Audacity did not copy your MP3 — it decoded it, let you edit the audio, then encoded a brand new MP3 on the way out. The new file is bigger because the bit rate it was encoded at is higher than the bit rate of the original. That is Audacity's own explanation, stated in one sentence in its MP3 export FAQ.
What makes this confusing is that two official pages give two different answers for what Audacity's default bit rate is. Sort that out and the size jump stops looking like a bug.
What this actually means
Audacity's FAQ on MP3 export problems addresses this case directly: "If you import an MP3 file into Audacity, then after export you find it is much larger than before, this is because the bit rate you exported it at is higher than the bit rate of the original file."
Bit rate is the lever that controls MP3 file size, and the same FAQ states the relationship plainly while discussing a different problem: "The higher the bit rate you export at, the larger the file will be."
Nothing about your edit did this. An MP3 cannot be edited in place — the import decodes it, and the export runs the LAME encoder over whatever is on the timeline using whatever settings the export dialog currently holds. Those settings have no knowledge of what the imported file was encoded at.
The fix: choose the bit rate deliberately
Open File > Export Audio, choose MP3 from the Format dropdown, and look at Bit Rate Mode. Audacity's MP3 Export Options page describes four:
- Preset — "Default setting, this offers the various Quality settings recommended by the LAME developers for encoding music."
- Variable — "continuously varies the bit rate used according to the complexity of the sound... The settings range from 0 (highest quality) to 9 (smallest file size)."
- Average — "sets a known, average bit rate for the file, but within this average permits some fluctuations"; it "offers bit rates from 8 kbps to 320 kbps."
- Constant — "sets a constant bit rate for the encoding, regardless of its complexity... this usually gives the worst, least consistent quality for a given file size, but that size itself is completely predictable."
If predictable size is what you are after, Constant is the mode that gives it to you. Audacity recommends it for streaming — "it ensures the audio cannot stutter due to sudden peaks in the bit rate" — and for podcasts, where the same page says "'CBR' (Constant Bit Rate) is recommended, and not 'VBR' (Variable Bit Rate)." It gives usable rungs: "96 kbps CBR mono can give excellent quality for voice", "128 kbps CBR stereo can give reasonable quality where stereo is required", "192 kbps CBR stereo can give very good quality where stereo is required."
Do not simply match the original file's bit rate
This is the instinctive fix, and Audacity's FAQ argues against it: "If you export a low bit rate MP3 at the same bit rate as the original you will be degrading its quality much more than if you exported it at a higher bit rate."
Re-encoding lossy audio at the same low rate as the source is not a neutral round trip. Audacity states the trade rather than pretending the bigger file is free: "The penalty of the higher bit rate is that the new file would be larger and that file merge tools might no longer be able to join the new MP3 to others that had the original bit rate."
So: if the file has to slot into a set of existing MP3s, match the rate and accept the quality hit. Otherwise, let it be bigger.
The durable fix: stop editing in MP3
The larger-file problem and the quality-loss problem share one root, and the manual is blunt about it: "If you import an MP3 into Audacity, edit it then export it as an MP3, you will lose quality twice, once in the original MP3 encoding of the imported audio, then again when you export it from Audacity as MP3. So never do production work in MP3, instead work with a lossless format such as WAV, AIFF or FLAC and for final use then export to MP3."
Converting your source to WAV before you import it means every edit, effect and re-save happens losslessly, and you encode to MP3 exactly once — at a bit rate you chose.
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
That runs in your browser tab; the file is not uploaded anywhere. The MP3 to WAV converter is the same job as a standalone step, and WAV to MP3 handles the final encode if you would rather skip the export dialog.
Why the bit rate you read about does not match what you see
Here is the part that sends people in circles. The FAQ page that explains your oversized file says: "Audacity defaults to 128 kbps bit rate then remembers the last used bit rate if you change it."
But the MP3 Export Options page — the dedicated page for these settings — says: "The default MP3 encoding options in Audacity are 170 - 210 kbps VBR (preset 'Standard')." It confirms this in its preset list: "Standard: Very good quality encoding. This is the default setting in Audacity."
On that page, 128 kbps appears somewhere else entirely, as the default within Constant mode: "the default rate (128 kbps) gives you a known file size of about 1 MB per minute of mono or stereo audio."
Two pages, two numbers, both live. Go by the dedicated options page: out of the box you are on Preset mode at Standard quality, which is 170–210 kbps VBR. If your original was a 128 kbps download, the defaults alone are enough to produce a noticeably larger file — exactly the situation the FAQ describes while quoting a different default.
The FAQ's other clause is easy to skim past: Audacity "remembers the last used bit rate if you change it." A rate you picked once is still sitting there for the next project.
Other reasons the file came out larger than expected
- You are on the Excessive preset. It "has a constant bit rate of 320 kbps, which is equivalent to 2.4 MB per minute" — and the manual calls it "generally considered to be overkill as there is little if any difference in sound quality from the 'Extreme' preset, but with much greater file size."
- Preset and Variable modes report ranges, not fixed numbers. "The bit rates are absolute for average and constant bit rate modes, but expressed as a range for the Variable and Preset modes (except for the Excessive preset)." So the same setting produces different sizes on different material.
- Your source is mono. For Variable mode the manual notes: "For a mono track, the achieved bit rate for each setting will usually be below the range indicated above." Separately, MP3 export "will always use joint stereo for the exported audio file."
- A third page calls 128 kbps CBR the default too. Audacity's AAC export page compares quality against "an MP3 at Audacity's default 128 kbps constant bit rate." If cross-checking pages got you nowhere, this is why.
How to tell it worked
Re-export, then compare the file size on disk against the original — not the waveform, which tells you nothing about encoding. If the new file is still larger and the bit rate you chose is the one you wanted, it is behaving correctly: a higher rate means a bigger file, and the manual treats that as the price of not degrading the audio further.
FAQ
Why is my exported MP3 bigger than the one I imported into Audacity? Because the export bit rate is higher than the original file's bit rate. Audacity's MP3 export FAQ names this as the cause outright. Audacity does not read the imported file's bit rate and reuse it.
Should I export at the same bit rate as the original to keep the size down? Only if you need to. The manual warns that exporting a low bit rate MP3 at the original's bit rate degrades quality much more than exporting at a higher rate. The stated reason to match anyway is compatibility with file merge tools that join MP3s sharing a bit rate.
What bit rate does Audacity actually use by default? The MP3 Export Options page says the defaults are 170–210 kbps VBR, the "Standard" preset. The MP3 export FAQ and the AAC page both refer to 128 kbps instead — on the options page, 128 kbps is the default inside Constant mode. Treat the dedicated options page as the reference.
Does re-exporting an MP3 lose quality even though the file got bigger? Yes. The manual says you lose quality twice — once in the original encoding, once on export — and advises never doing production work in MP3, using WAV, AIFF or FLAC instead and exporting to MP3 only at the end.
How do I avoid this happening at all? Convert to a lossless format before you edit, keep that as your working copy, and encode to MP3 once with a bit rate you picked on purpose. All Clapr tools run in the browser, so nothing is uploaded on the way.
Bit rate is behind most "why is this file so big" questions in every editor, not just Audacity — the same logic applied to video is in what the bitrate setting actually does. For more export failures and their causes, browse the fix guides.
All quotations from the Audacity Manual, checked 16 September 2026: FAQ: MP3 export problems, MP3 Export Options, Export Formats supported by Audacity.
