I'm curious as to why don't your shoulders gain weight?

Fat cells: If by gaining weight, you mean gaining fat, shoulders' structure consist of bones and muscle, very little fatty cells. Unless someone is on high dose steroids which can cause a hunch back look. When one gains fat, the common areas that have a lot of fat cells to expand are abdomen, breasts and buttocks. The number of fat cells are fixed but they get larger. Muscle weighs more than fat same volume.