Technically, It is most likely to be that the bros are fuzzy. Their bones would be covered in velvet (Ever seen male deer with fuzzy antlers? Yeah, that's called velvet, it helps to supply the bone with blood before it hardens to antlers), possibly to help run blood (Or whatever the monster equivalent is) to the different parts of the bone, keep the skeleton warm in cold weather, have nerve endings so that they can 'feel' in a sense, or could even be used as a defence mechanism if they can fluff it up by themselves, making them look bigger.TLR: Skeleton's fuz could be like deer Velvet???