Thats skinny one looks muscular to me. I prefer the word toned though.
I have many things to do, but I shall be back, and hopefully give some good critique.
Edit:
Ok lets get started.
Well Toned skinny Dude:
Well he's head is to wide in proportion to his body, I think head size ratio width wise is about 1/3 to the body, where 2/3's are 1/3 a shoulder each. (these are not accurate ratio's)
Normal sized Toned dude:
he's probs the only one that looks about right.
Muscle man:
Well firstly, the abdominals don't look right, and the rib cage should be just about aligned with the hips.(again this is not accurate, as the rib cage can differ depending on weight and many other factors, and on the average person, from the upper part of the rib cage to the lower, you might notice that that move slightly toward the inner part of the hips.) keeping in mind all factors. So this needs to be a bit wider by the abdominals and again the head on this look awkward.
Fat man:
Well he looks good until you get to the hips, where the transition between stomach and The Elvis pelvis are and isn't good at all.
Overall:
the legs are ok on all of them but need a bit of something, I can't get my finger on it though.