But the thing is that we *don't* compute it ourselves -- the dimensions and file size are already provided with each graphic. It's a function of an operating system to provide the size(bytes) and dimensions of a file.

As I mentioned before, there is another variable involved -- the resolution/DPI. That's why it isn't possible to compute the file size JUST knowing the height and width. If you do know the resolution/DPI, then you can use the calculator I posted a link to.