Many psych meds cause weight gain by drastically increasing appetite. Example: skinny alcoholic stops drinking and goes to rehab, gets put on a slew of psych meds and balloons into a creature you can't recognize. "Sally, is that you?" sort of thing. When you see a small-medium fat that looks like they were recently inflated with air you can be relatively sure they are on some type of psych med. Combine this side affect of the medication with lack of willpower and you get a fatty.