Video Compression Explained
Video compression removes repeated and less noticeable information so moving pictures can be stored and transferred with far less data.
Frames are not all stored independently
Modern codecs keep complete reference pictures at intervals and describe changes between them. Sudden cuts and chaotic motion are therefore harder to compress.
The container is not the codec
MP4 and MOV are containers. H.264, H.265 and AV1 are codecs. A file extension alone cannot reveal whether every browser can decode the streams inside.
Why estimates remain estimates
A constant-quality encode adapts to the content. Even target-bitrate modes have container overhead and encoder behavior, so exact byte counts can vary.