What actually determines video file size

The only formula that matters

File size is approximately bitrate multiplied by duration. A video at 8 megabits per second running 60 seconds is about 60 megabytes, regardless of whether it is 1080p or 4K.

That is why a heavily compressed 4K clip can genuinely be smaller than a high-bitrate 1080p one. Resolution does not set the size; it sets how much bitrate you need before the picture starts falling apart.

What resolution really does

More pixels need more data to stay sharp. Roughly, 1080p needs 8–12 Mbps for clean H.264, and 4K needs four times that for equivalent per-pixel quality.

Push 4K through a 1080p-sized bitrate and you get a large, soft, blocky picture - worse than simply delivering 1080p properly. Downscaling is usually the better lever.

Frame rate and content complexity

Doubling frame rate from 30 to 60 roughly increases the bitrate needed by 50%, not 100%, because consecutive frames are more similar.

Content matters more than people expect. A static talking head compresses extremely well; confetti, rain, water and fast camera movement defeat the encoder's ability to predict frames and inflate the file substantially at the same settings.

Codec is the free win

H.265 (HEVC) and VP9 achieve roughly the same visual quality as H.264 at about half the bitrate. AV1 is better still.

The cost is compatibility, not quality - which is exactly the trade-off discussed in the MP4 versus WebM comparison. When the destination accepts it, changing codec is the only lever that reduces size without reducing quality.

Practical order of operations

Downscale resolution if it exceeds what the destination displays. Nothing on a phone feed benefits from 4K.

Then lower bitrate until quality visibly suffers, and step back one notch.

Only then consider dropping frame rate - it changes the feel of the footage and viewers notice it more readily than a modest bitrate reduction.