As we know, we're all free to look how we want in SL but when the herd instinct drives fashion trends like huge hips and tiny waists, or in the case of males, huge shoulders and biceps, that leaves 'normal' avatars looking like the odd ones out.
Creating a realistic shape is not easy for everyone, even with a system avatar. There must be quite a few avatars that are the result of the owner's lack of perception of scale and proportion. For instance a lot of avatars, both female and male, have their heads too small, while a lot of female ones have arms that are much too short with microscopic hands. Neither of these are likely to be fashion trends as such.