>>1
It's because it's harder for extremely fat people to wash themselves, as well as wipe after going to the bathroom, due to the extra fat obstructing the reach of their arms, which on a person of average weight, would suffice for reaching their back/butt/etc. Also, fat people who are relatively self-conscious about their weight don't like taking their clothes off, whether it be to take a shower, or whatever.
I'm not joking. Although the majority of fat people I've met don't smell.