I've never taken a life-drawing class, but it would be pure pleasure to walk into the studio and see any one of these men below the cut as models. With a fat man as model, the artists have to take into account the three-dimensional weight and mass of his body. Also, since it's so unusual to see fat men unclothed (as in media, outside of slapstick comedy like the Jackass movies, for instance), the artists are compelled to draw what they see, not some idealized male form.
Unknown model drawn by Rennie Behrend, a St. Louis art professor:
Model Michael Schmidt (whom I wrote about earlier), painted by Katherine Sly:
The four below were taken by Michelle Raspanti, of male model Artclasmodl. (The link is only viewable by flickr members, sorry.)