Resize a video to 720x1280, the 9:16 vertical frame at half the pixel count of 1080x1920.
720x1280 against 1080x1920
The shape is the same. The difference is pixel count: 921,600 against 2,073,600, or 44%. At the same quality setting the file lands at roughly half the size, which is the entire reason to pick it.
Use it when the constraint is an upload limit, a messaging app, an email attachment, or an older device. Use 1080x1920 when the footage carries text, fine texture, or screen content that has to stay readable.
Feed platforms re-encode everything they receive, so the gap between the two after their pass is smaller than the raw pixel counts suggest. The gap in what you upload is not.
Scaling behaviour
A 1080x1920 source scales down cleanly to 720x1280: both sides divide by exactly 1.5 and nothing is cut or padded.
A 1920x1080 landscape source is a different job: 656 pixels come off each side, keeping a 608x1080 slice, which is then scaled to 720x1280. To keep the full width instead, pad it in the video cropper.
Do not upscale into it
Going from 720x1280 back up to 1080x1920 later adds pixels without adding detail. If there is any chance the clip is needed at full size, keep a 1080x1920 master and export 720x1280 as a copy.