Why fat kids have low confidence and low self-esteem ?

Well, i really don't know why i have seen many big kids were getting teased and beaten up by smaller kids think is they don't fight back they only getting scared by others, especially the fattest one ! as i knew at school on my cl theres a boy his size is bigger than our cl mates around 4 times bigger but his totally body fat and our ages is 15, but he looks older than his age, the problem is most of guys humiliating him so badly every time where he had accepted and he does cry ! even thou he was beaten up by girls ! honestly i never teased him but i didn't like to see very big guy got beaten by small guy or a skinny girl that knocked him down and sit on the top of him ! it looked so awful, because on other hand he may beat them all only if he stand up by him self because hes too big no ?