Why are guys jerks? ?

So there's this guy.. a very big guy.. the waddles when he walks kind of big.. he is dating my bf's ex.. he and I have never met.. or talked..but the big jerk keeps posting mean things about me on facebook.. i am no super model but I'm a heck of a lot smaller and better looking then his duck waddling butt.. so here is my question ... why is it ok for guys to be big tubs of rude ill behaved lard.. but heavens forbid a chick is a little thick?