Poor diet. Fatties tend to eat more cured meats and other food which potentially carry chemicals that interfere with the normal cell division process. There are a number of redundant protections in cells which self destruct the thing during cell division if the copied DNA reads wrong. Chemicals which neuter these protections are many and Hamplanets get a lot of dietary exposure by eating like crap and eating lots of crap. Being fat doesn't make one's organs bigger so organs like the colon in the obese can stew in far greater amounts of this shit than a shitlord's colon will. The dose makes the poison, but once you toss moderation out the windows otherwise benign stuff can start looking like poison. To top it off in poorly controlled beetus they body starts making and accumulating more poisons of its own. Then with kidney failure you cripple a major route for expelling posions...