How Video Compression Works

File size starts with bitrate and duration

A video's bitrate describes how much data is used over time. At the same duration, a lower bitrate usually creates a smaller file. That is why exact-size compression begins by calculating how many bits per second fit inside the requested target.

Audio also consumes part of the target. A practical compressor reserves space for audio and container overhead before assigning the remaining budget to video.

Resolution is not the same as quality

A 1080p frame contains more pixels than a 720p frame. If both are forced into a very low bitrate, the 1080p version can look worse because there is not enough data to describe all of those pixels well.

For aggressive compression, lowering resolution can produce a cleaner-looking result than keeping a high resolution at an extremely low bitrate.

Why exact sizes are approximate

Video encoders distribute data differently depending on motion and detail. A talking-head clip and a fast sports clip may not land at exactly the same size with identical settings. FitMyVideo therefore keeps a conservative safety margin so the first encode aims below the selected cap without routinely repeating the entire encode.