Dimensions count pixels. File size counts stored bytes. Format, quality, metadata, and image complexity connect them, but not with a fixed formula.
Dimensions
A 1600 by 900 image has 1.44 million pixels. Its displayed size may be smaller or larger depending on CSS, screen density, print settings, or the app showing it.
File size
A 1600 by 900 flat graphic can compress very differently from a noisy night photograph. File bytes include compressed pixels, metadata, color profiles, and container overhead.
Which tool to choose
Use Resize when a platform requires exact width and height. Use Exact Size when it requires a KB or MB cap. Use both when the destination specifies both constraints.