Resolution specifies pixel count, not bit budget
1080p means 1920×1080 pixels per frame. It does not dictate how many bits the encoder spends compressing those pixels.
The encoder determines file size
Codec efficiency (H.264 vs H.265 vs AV1), scene complexity, motion intensity, and bitrate settings dictate the file size.
Use bitrate when estimating storage
When you know target bitrates, use the File Size Calculator. If you have an upload size limit, work backwards using the Bitrate Calculator.