They like making fun of him on this site because he and Liam have issues.
And it really doesn't have to do with actual appearance. I may call myself "Big" but I'm actually underweight for my height and age. So you can call someone fat but it doesn't mean their overweight. It's basically just random insulting.
As for the rant, I do find it annoying how there are idiots out there who don't seem to understand what is going on.