confidence knocked by strangers

So today I thought I'd brave it and go for a small run on my own before doing a longer run later. Whilst going through a pretty quiet area I was shouted at by 2 passers by. One called me a fat slob and the other called me a fat bitch. Bearing in mind I'm trying to lose weight for my future wedding this was a pretty big confidence knock. I've been bigger in my life and have heard much worse, but why would people go out of their way to make comments like that? To make themselves feel better? To just have a laugh? Or just feel like being cruel today? I don't know, my point is, no matter what's someone size is, if they are clearly doing something to maintain a healthy life don't go out of your way to make them feel rubbish. Either say nothing or encourage them.