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An MP4 can grow after FFmpeg re-encoding because MP4 is only a container: file size depends mainly on the video and audio streams inside it. The new encode may use a higher bitrate or quality target, a less compression-efficient codec or preset, more pixels or frames per second, or additional streams. Compare the streams and encoding settings in both files to find the cause; the .mp4 extension alone cannot explain the size change.
What determines the file size?
For a video of a given duration, higher average bitrate usually means a larger file. The video stream is often the largest part, but audio and any retained subtitle, data, or extra audio streams contribute too. Re-encoding does not inherently shrink a file: it replaces the original streams with new ones, and the new settings may spend more bits to preserve quality.
A source already compressed with an efficient codec can also become larger when converted to a less efficient codec or settings. Even if the result looks similar, its encoder may have used more data. Compare the actual streams rather than judging by the container or appearance alone.
Find what changed with ffprobe
Run ffprobe on each file and compare the output. This command lists stream and format details in a readable form:
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ffprobe -hide_banner -i input.mp4
Repeat it for the re-encoded file. Check the following:
- Video codec and bitrate: Compare the codec and reported average bitrate. A higher video bitrate is a common reason for a larger output.
- Dimensions and frame rate: Check whether the output has more pixels or frames per second than the source. Upscaling or increasing frame rate can increase the amount of video data.
- Duration: Confirm both files cover the same length of footage.
- Audio: Compare codec, channel count, sample rate, and bitrate. Audio re-encoding or retaining more channels can add size.
- Stream count: Check whether subtitles, extra audio tracks, or data streams were copied into the output.
- Encoding command: Review
-c:v,-crf,-preset,-b:v, scaling or frame-rate filters, and audio options.
Stream bitrates and the total file size are related but not identical: container overhead and other streams also take space. If the video bitrate rose while duration stayed the same, that is a strong explanation for the increase; if not, inspect audio and stream count too.
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How CRF, preset, and bitrate affect an x264 encode
CRF controls the quality–size tradeoff
When using FFmpeg’s libx264 encoder with CRF rate control, a lower CRF targets higher quality and commonly produces a higher bitrate and larger file. A higher CRF targets lower quality and smaller output. The resulting size depends on the footage, source quality, encoder build, and settings, so no single CRF value predicts a particular file size or suits every video.
Preset changes compression efficiency
The -preset setting controls the encoder’s speed-versus-compression tradeoff. Faster presets can be less compression-efficient at similar visual quality, while slower encoding can improve compression efficiency at the cost of more encoding time. A faster preset is not a guarantee of a smaller file.
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Explicit bitrate targets constrain the rate
If the command specifies -b:v, it is asking for a video bitrate target rather than relying only on CRF. Check whether that target is higher than the source’s average video bitrate. A target is not the same thing as a guaranteed final file size; duration, audio, other streams, and the rate-control workflow matter too.
Choose settings for a YouTube upload
YouTube’s current upload encoding recommendations, accessed in 2026, list MP4 as a container and H.264 as a video codec, and advise keeping the frame rate used for recording. For SDR uploads, the published bitrate figures are reference recommendations, not hard caps or requirements for every export:
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| Resolution and frame rate | YouTube SDR bitrate recommendation |
|---|---|
| 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 are upload guidance, not instructions to cap every encode at those bitrates or a promise that a video at a listed value will have a particular size. YouTube says it re-encodes uploaded video files as necessary, so the uploaded file’s size does not determine the size or encoding of the playback rendition viewers receive.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Reduce the size without changing more than necessary
- Keep the source dimensions and frame rate unless they are unnecessarily high for the intended upload. Avoid upscaling or changing frame rate merely to make the file more YouTube-ready.
- Check the codec and rate control in the FFmpeg command. With
libx264CRF encoding, raise CRF in small steps and inspect each output. Expect a size-versus-visible-quality tradeoff; review the result before replacing the original. - Consider a slower preset if more encoding time is acceptable and compression efficiency matters. It may reduce size at similar quality, but the outcome depends on the material and settings.
- Inspect audio and extra streams before removing or changing them. If they account for little of the total, adjusting them will not solve a large video-bitrate increase.
- Use an explicit bitrate target when a predictable average bitrate is required, with an appropriate rate-control workflow. Compare the chosen target with the source and intended use rather than treating YouTube’s recommendations as mandatory limits.
For example, a simple x264 CRF encode that keeps the source dimensions and frame rate can be expressed as ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium -c:a copy output.mp4. This is an example command, not a universal size target: CRF 23 may produce different results for different footage, and copying audio is appropriate only if the existing audio is suitable for the output. Compare the resulting file with the original and adjust one setting at a time.
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Why a larger upload is not necessarily a YouTube problem
YouTube’s published recommendations are for upload encoding, while YouTube processes uploaded files for playback. YouTube states: “YouTube will still accept your video content and then re-encode your video files as necessary.” A larger local MP4 therefore does not establish that viewers will receive a larger playback file or a better-looking rendition.
Troubleshoot common size increases
- The video bitrate is higher: The CRF may be lower, the explicit
-b:vtarget may be high, or the new encode may be less efficient. Review rate control and preset; if size reduction is the goal, raise CRF gradually and check visible quality. - The output is larger in resolution or frame rate: Remove unintended scaling or frame-rate changes and preserve the recording settings unless there is a reason to alter them.
- The source used a different codec: Compare the source codec with the output encoder. A conversion to a less efficient codec or settings can increase size even when the result looks similar.
- Audio or stream count increased: Look for re-encoded audio, extra channels, subtitles, or additional tracks. Change or omit streams only when they are not needed.
- The file is larger despite similar-looking video: Similar appearance does not mean equal bitrate or compression efficiency. Compare stream-level figures and make a controlled settings change rather than relying on visual similarity alone.
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