How video bitrate affects file size
Bitrate is the amount of media data used per second; duration turns that rate into a final file size.
The simple formula
A video file gets larger as bitrate or duration increases. Audio also contributes to the final size, although video bitrate is usually the larger part for normal clips.
Why target size is not exact
Encoders make decisions scene by scene. A simple slideshow-like video may compress easily, while fast motion, noise, or screen text may need more data. That is why browser-based target-size output can land slightly above or below the requested size.
Resolution still matters
Higher resolution gives the encoder more pixels to describe. If the output only needs to be viewed in a chat or email preview, reducing width can make a lower bitrate look cleaner.