I tend to squeeze away from people on public transit regardless of their girth, but the difference between scooting away from a shitlord/lady and scooting away from the fatty is that one I can avoid touching one completely, and the other I can only avoid touching....less.  Also, humans tend to be conscientious about how much space they take up while fats interpret shifting away from them as "Hey, more space for my massive gut to seep into! Don't mind if I do teehee~"