Pixels are what bits get spent on. Dropping 1080p to 720p removes 56% of the pixels in every frame; 1080p to 480p removes 80%. The savings in megabytes are large but not perfectly proportional, because the encoder also spends bits on motion and detail.
The width is computed from your chosen height and rounded to an even number because video codecs work in 2-pixel blocks. It sounds pedantic, but it is the difference between a file every player accepts and one that errors out halfway through an upload.
If a platform will show your video small, resizing before upload beats letting the platform do it: you control the scaler and the upload finishes sooner. But if someone asks for 1080p and you have 720p, send the 720p: an upscale would only look worse.