An MP4 file can grow after FFmpeg re-encoding because MP4 is only a container: the encoded streams inside it determine most of its size. A higher bitrate or quality target, a less efficient codec, more pixels or frames per second, or larger audio and extra streams can outweigh any savings from re-encoding. Compare the source and output settings before changing the command; the extension alone cannot tell you which file should be smaller.
What determines the size of an MP4?
For a given duration, the combined bitrates of the video, audio, and any other streams largely determine the file size. A rough estimate is:
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File size in megabytes ≈ total bitrate in megabits per second × duration in seconds ÷ 8.
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Why can FFmpeg make the file larger?
The output bitrate or quality target is higher
With the libx264 encoder, -crf sets a quality-versus-size tradeoff. A lower CRF generally aims for higher quality and commonly uses more bitrate; a higher CRF generally means lower quality and a smaller file. The exact size depends on the video content, source quality, encoder build, and settings, so no single CRF guarantees a particular file size.
An explicit -b:v target can also lead to a larger output if it is higher than the source’s average video bitrate. Check whether your command uses CRF, a bitrate target, or a combination of rate-control options.
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The codec or preset compresses less efficiently
The source may use a more efficient video codec than the output’s H.264 encode. A visually similar H.264 file can therefore be larger. The libx264 -preset also affects compression efficiency: faster presets can produce larger files than slower ones at a similar quality target. A slower preset costs more encoding time; it does not promise an exact size.
Resolution or frame rate increased
Scaling to more pixels or encoding more frames per second can increase the amount of video data. Compare the source and output dimensions and frame rates, and inspect any -vf filters or frame-rate options in the command. If you did not intend to upscale or change the frame rate, remove or correct that setting.
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Audio or additional streams add data
Audio may have been re-encoded at a higher bitrate, changed to more channels, or converted to a different sample rate. Subtitles, additional audio tracks, and data streams can also contribute to the output. Check whether audio was copied or re-encoded and compare the number and types of streams in both files.
The source was already compressed
Re-encoding a compressed video does not automatically make it smaller. If the new settings are not more space-efficient, the output may grow—and re-encoding can also reduce quality. Keep the original when possible, particularly if you may need to make another export later.
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Compare the source and output instead of guessing
Use ffprobe or a media-information tool to inspect both files. Compare the following, then match any difference to the relevant FFmpeg options:
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- Video: codec and encoder, average bitrate, dimensions, and frame rate.
- Audio: codec, bitrate, channel count, and sample rate; note whether it was copied or re-encoded.
- Streams: total number and types, including subtitle, extra audio, and data streams.
- Command: inspect
-c:v,-crf,-preset,-b:v, scaling or frame-rate filters, and audio options.
Duration matters when comparing total file sizes: a longer output can be larger even at the same average bitrate. Average bitrate is often a more useful comparison when durations differ.
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How to reduce size for a YouTube upload
- Keep the intended dimensions and frame rate. Preserve the source resolution and recording frame rate unless they are unnecessarily high for the video. Check scaling and frame-rate filters so they do not increase either by accident.
- Choose the video codec and rate control deliberately. YouTube lists MP4 as a container and H.264 as a video codec in its upload recommendations. For libx264, raise CRF in small steps if a smaller file is more important than retaining the same quality, and inspect the result. If you need a predictable average bitrate or size, use an explicit bitrate target and an appropriate rate-control workflow.
- Choose a preset based on your encoding-time tradeoff. A slower preset may improve compression efficiency when similar visual quality is the goal and you can wait longer. Check the resulting quality and size rather than assuming a preset guarantees either.
- Review audio and stream mapping. Avoid needlessly increasing audio bitrate or retaining tracks that are not required for the upload. Keep any subtitles or extra audio that your project actually needs.
- Compare the result before replacing the original. Check file size, average bitrate, dimensions, frame rate, audio, and visible quality. Adjust one setting at a time so you can identify what caused a change.
Use YouTube’s bitrate figures as guidance, not a file-size cap
YouTube’s current upload encoding recommendations, accessed in 2026, give these SDR video bitrate figures:
| Resolution and frame rate | Recommended video 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, not hard caps or a prescribed FFmpeg command. YouTube says it re-encodes uploaded video files as necessary, so your upload’s size does not guarantee the size or quality of the playback rendition viewers receive. See YouTube’s video and audio formatting specifications.
Common causes and fixes
| What you find | Likely explanation | What to check or change |
|---|---|---|
| Output video bitrate is higher | Higher CRF quality target tradeoff, explicit bitrate, or other rate-control settings. | Review -crf and -b:v; raise CRF gradually if appropriate, or set a deliberate bitrate target. |
| Output has more pixels or frames per second | A scaling or frame-rate setting changed the video. | Compare dimensions and frame rate; inspect and correct filters and frame-rate options. |
| Video looks similar but output is bigger | The source codec or its settings may be more compression-efficient, or the output preset may be faster. | Compare codecs and presets; consider a slower preset if encoding time allows, then review quality. |
| Audio bitrate, channels, or stream count increased | Audio was re-encoded differently, or extra streams were retained. | Compare audio properties and stream lists; adjust audio or mapping options to match your needs. |
| Same settings seem to produce different sizes | Content complexity, source quality, or encoder build can affect the result. | Do not treat CRF as an exact-size control; compare the actual output and use bitrate-based rate control when predictable average size matters. |
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If your goal is to keep a pre-recorded YouTube stream running 24/7, StreamNeo is a separate option from preparing an upload with FFmpeg: upload a recording or build a playlist, add your YouTube stream key, and go live. It loops uploaded videos from the cloud, so nothing has to stay on at home. The video streams as uploaded, up to 4K 60fps, at one flat price per slot; StreamNeo automatically recovers if YouTube drops the stream. The first day is free with no card. Monthly is $9.99 per month. Learn more at StreamNeo, or start your free first day.
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