In this episode the fact that Stan is not in the best of shape is drawn to his attention. He goes nuts, going on a crazy exercise program and getting an abusive personal trainer. However Stan gets fatter and fatter as his family seem to be conspiring to make him even fatter. He eventually gets suspended from the CIA when he collapses on an assault course.
And then we get the twist.
Stan does indeed have a "weight problem", but that problem is Anorexia. His body image is distorted, he is really as thin as a rake and what is more, his personal trainer is a hallucination.
By making Stan the sufferer they get to explore the perceptions of Anorexia, more than once Stan is referred to as a teenage girl by the support group. The way the episode works also draws attention to the way that the disease works and, all in all, this is one of the better treatments of Anorexia in mainstream media. Well worth seeing, both as an episode in its own right and as a warning as to the dangers of this disease.