File-size problems
Compression made the file bigger? Why it happens and the fix
Short answer
A file grows after compression when the new settings ask for more bits than the source used: a CRF lower than the source's effective quality, a fixed bitrate above the original, an intermediate or lossless codec, or PCM audio. Check both files' bitrates with ffprobe and raise CRF until the result is smaller; if the source was already efficient, keep it.
Key takeaways
- CRF is a quality target, not a size target; a low CRF can exceed the source's bitrate.
- A source encoded efficiently (for example HEVC) can grow when converted to H.264 at the same quality.
- Lossless or PCM audio and intermediate codecs are large.
- If the file is already small and clean, do not re-encode it.
Find your case
| What you see | What it usually means | Do this |
|---|---|---|
| H.264 output larger than HEVC source | H.264 needs more bits for the same quality | Raise CRF or keep HEVC where accepted (step 2) |
| Output larger after setting a high bitrate | Bitrate above the source | Use CRF instead (step 2) |
| Audio is a large share | PCM or very high audio bitrate | Use AAC 128 to 160 kbps (step 3) |
| Phone video re-encoded and grew | Phones encode efficiently | Skip re-encoding (step 4) |
Fix it step by step
Compare the numbers
ffprobe -hide_banner in.mp4and the same for out.mp4: compare bit rate, codec and duration. A bigger bitrate or a different codec explains the growth.Use CRF, with a ceiling above 23
ffmpeg -i in.mp4 -c:v libx264 -crf 26 -preset slow out.mp4. A higher CRF lowers bitrate; +6 roughly halves it, per the FFmpeg wiki.Fix audio separately
-c:a aac -b:a 128kreplaces PCM or high-rate audio. Audio is rarely the biggest part of a video file, but it can be for screen recordings.Keep the original when it is already small
If the source is efficient and under the limit, upload it as is; re-encoding costs quality and may add size.
Typical reasons side by side
| Change | Effect on size |
|---|---|
| CRF down by 6 | About doubles |
| CRF up by 6 | About halves |
| HEVC to H.264 at the same quality | Usually larger |
| AAC to PCM | Much larger audio |
Check your file
Extension and MIME type are labels, not proof of the container. Dimensions and duration come from this browser's media reader. This does not detect codecs, bitrate, corruption or platform upload compatibility. Files and measurements are not sent anywhere.
Frequently asked questions
Why is my compressed video bigger?
The new encode asks for more bits than the original: lower CRF, higher bitrate, a less efficient codec or lossless audio.
Does converting HEVC to H.264 increase size?
Often yes, because H.264 needs more bits for similar quality.
How do I make it smaller?
Raise CRF, use a slower preset and re-encode audio to AAC.
Sources
- FFmpeg Wiki: Encode/H.264 (CRF, presets) (official, checked 2026-10-06)
Reviewed 2026-10-06. Steps not attributed to a source are general video practice or computed above.