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An MP4 file can grow after FFmpeg re-encoding because MP4 is only the container: the new video or audio streams may use higher bitrates, a less efficient codec, larger dimensions or frame rate, or additional streams. Compare the streams and the FFmpeg options—not just the filename extension—to find the cause.
What determines an MP4 file’s size?
Most of an MP4’s size comes from its encoded video and audio. A re-encode creates new streams, and there is no rule that those streams must be smaller than the originals. A higher video bitrate, a higher-quality target, or a less compression-efficient codec can make the result larger even when the picture looks similar.
With FFmpeg’s libx264 encoder, -crf sets a quality-versus-size tradeoff: lower CRF generally means higher quality and often a larger file; higher CRF generally means lower quality and a smaller file. The -preset also affects compression efficiency. Faster presets can produce larger files than slower presets at comparable visual quality, in exchange for less encoding time.
Other changes can add size too: scaling up the image, increasing the frame rate, re-encoding audio at a higher bitrate, or keeping extra audio, subtitle, or data streams. Re-encoding an already compressed source does not inherently reduce its size.
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Compare the source and output before changing settings
Inspect both files with ffprobe or a media-information utility. Compare the individual streams, because total file size alone does not reveal which part grew.
- Video codec and encoder: Check whether the source used a more efficient codec than the output.
- Video bitrate: Compare the average bitrate, not just the codec name. A higher output bitrate is a common reason for a larger file.
- Dimensions and frame rate: Look for an increase in resolution or frames per second.
- Duration: Confirm that the output is not longer than the source.
- Audio: Compare codec, channel count, sample rate, and bitrate. Check whether the audio was copied or re-encoded.
- Other streams: See whether subtitles, alternate audio, or data streams were included in the output.
- FFmpeg options: Review
-c:v,-crf,-preset,-b:v, scaling or frame-rate filters, and audio options.
This comparison narrows the cause to a specific stream or setting. If the output has a higher video bitrate, for example, adjusting video rate control is more relevant than changing the MP4 container.
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How to reduce size without blindly sacrificing quality
- Keep resolution and frame rate unchanged unless they are unnecessarily high. Preserving the source’s dimensions and recording frame rate avoids adding pixels or frames without a clear need.
- For libx264, raise CRF in small increments and review each result. A higher CRF usually reduces size, but also lowers quality. There is no universally correct CRF: the result depends on the content, source quality, encoder build, and acceptable visual quality.
- Try a slower preset if encoding time is acceptable. A slower preset can improve compression efficiency at similar quality, though encoding takes longer.
- Set an explicit bitrate when predictable bitrate or average size matters. Use a rate-control workflow suited to the target. A bitrate target is a different goal from asking for a particular visual quality, so check the resulting picture as well as the size.
- Recheck audio and streams. If audio was re-encoded or unnecessary streams were retained, address those choices deliberately rather than assuming video settings explain the whole increase.
What YouTube’s upload recommendations do—and do not—mean
YouTube’s current upload encoding recommendations, accessed in 2026, list MP4 as a container and H.264 as a video codec, and advise uploading at the frame rate used for recording. For SDR uploads, the published reference bitrates include:
| Resolution and frame rate | YouTube reference 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 are upload recommendations for reference, not hard caps, guaranteed file sizes, or mandatory FFmpeg targets for every video. Use them as orientation when choosing an export, alongside the quality and compatibility you need. They are not YouTube Live streaming settings.
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YouTube also notes that it re-encodes uploaded video files as necessary: “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 bitrate of the playback rendition viewers receive.
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