Someone that is overweight and has very visible rolls is someone with a loaf,for instance if someone has rolls on lets say their back they have loafs on their back
"A Loaf" refers to a whole uncut bread, not a chunk. A loaf is the complete item, as removed from the oven. Bread in it's virgin state. (Western style bread I think, doesn't really work for unleavened breads eg. Pitta, Nan, Chappatti etc.).