An MP4 file can grow after FFmpeg re-encoding because MP4 is only a container: the encoded video and audio streams determine most of the size. A higher bitrate or quality target, a less efficient codec, larger dimensions or frame rate, or extra streams can outweigh any savings from re-encoding. Compare the source and output settings before changing the command; the “.mp4” extension alone says nothing about expected size.
What determines an MP4 file’s size?
A rough estimate for the main streams is: size in bytes ≈ total bitrate in bits per second × duration in seconds ÷ 8. Container overhead is usually smaller than the video stream, but audio and any retained subtitles or other streams add to the total. A new encode is not inherently smaller: it only becomes smaller if its encoded streams use fewer bits overall.
Re-encoding an already compressed source can also make a visually similar file larger. The source may use a more efficient codec or settings than the output, and another lossy encode cannot recover detail already discarded.
Compare the source and output before changing settings
Use ffprobe on both files, or inspect them in a media-information tool. For a concise stream and format summary, run:
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ffprobe -v error -show_entries format=duration,size,bit_rate:stream=index,codec_type,codec_name,profile,width,height,r_frame_rate,avg_frame_rate,bit_rate,sample_rate,channels -of json input.mp4
Replace input.mp4 with each filename. Some files or streams do not report every bitrate or metadata field; treat missing values as unknown, not zero. Compare these details:
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- Video codec and bitrate: Check whether the output codec is less efficient than the source and whether the output video bitrate is higher. Container-level bitrate includes all streams, so compare video with video where possible.
- Quality and rate-control options: Inspect the actual command for
-crfand-b:v. With libx264, a lower CRF targets higher quality and commonly produces a higher bitrate; a higher CRF targets lower quality and smaller output. CRF does not promise a particular file size. - Preset: Faster libx264 presets can compress less efficiently at similar visual quality. A slower preset may reduce size for comparable quality, at the cost of more encoding time.
- Dimensions and frame rate: Look for changes from scaling filters, frame-rate conversion, or different output settings. More pixels or frames per second can increase the amount of encoded data.
- Audio: Check codec, bitrate, channel count, and sample rate. Audio may have been re-encoded at a higher bitrate, changed to more channels, or retained alongside additional audio streams.
- Stream count: Check for subtitles, alternate audio, data, or other streams copied into the output. They contribute to the file even if the video itself did not grow.
Then inspect the full FFmpeg command, especially -c:v, -crf, -preset, -b:v, scaling or frame-rate filters, audio options, and any stream-mapping options. This comparison identifies what changed; without the source file and command, no exact cause or output size can be diagnosed.
How YouTube’s upload recommendations fit in
YouTube lists MP4 as a container and H.264 as a video codec, and recommends retaining the frame rate used for recording. Its bitrate figures are reference recommendations for uploads, not hard caps or guarantees of a particular file size. For SDR video, YouTube’s current upload encoding recommendations (accessed 2026) list:
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| Resolution and frame rate | Recommended bitrate |
|---|---|
| 1080p at 24, 25, or 30 fps | 8 Mbps |
| 1080p at 48, 50, or 60 fps | 12 Mbps |
| 720p at standard frame rate | 5 Mbps |
| 720p at high frame rate | 7.5 Mbps |
These figures help orient an upload export, but they are not instructions to cap every file at those bitrates. YouTube says, “YouTube will still accept your video content and then re-encode your video files as necessary.” The uploaded file’s size therefore does not determine the size or quality of the playback rendition viewers receive. See YouTube’s Video and audio formatting specifications.
How to reduce size without guessing
- Preserve the source dimensions and frame rate unless they are unnecessarily high for the intended video. Changing either can affect detail or motion as well as size.
- For libx264, raise CRF in small steps and review the output at representative scenes. Higher CRF usually means smaller output, but also lower quality; the right setting depends on the content and acceptable visual loss.
- Try a slower preset if encoding time allows. It can improve compression efficiency at similar quality, but it is not a guarantee of a smaller file in every case.
- Check audio and stream mapping. Avoid keeping unwanted alternate tracks or subtitles, and do not raise audio bitrate or channel count without a reason. If you need those streams, account for their size rather than treating them as video overhead.
- If size or average bitrate must be predictable, use explicit bitrate-based rate control. Choose a target appropriate to duration, quality, and delivery needs; CRF alone cannot guarantee a specific size.
- Compare the result visually and technically. Check the new file’s duration, resolution, frame rate, streams, and bitrate alongside representative footage. Smaller is not automatically better if the output has lost needed detail or compatibility.
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