(ZK, get ready for lots of flack. Weight is a very touchy subject)
Because calories don’t start burning in huge numbers until you start moving around really quick, for an extended lenght of time. So while the heavy person expends more effort to move, they do so slower, and stop more often.
Skinny people don’t keep losing weight because at a certain point, you’re only skin and bones (and some organs) and that does not burn off by excercise