Where the bytes go
A video file spends almost all its space on the picture track, and the encoder spends its budget per second of footage.
Resolution sets the floor
More pixels per frame means more bytes per frame. A 4K frame carries four times the pixels of 1080p before compression even starts.
Frame rate multiplies it
Sixty frames a second is twice the frames of thirty. Content with little motion between frames compresses the difference away cheaply; busy content does not.
Recording bitrate is the culprit
Modern phones record at bitrates that pass 10MB in seconds — which is why raw phone clips are the usual reason people need to reduce video size at all.
