A half-affectionate, half-disparaging* nickname for fat people. Its origins are two-fold:

1) Kind of like a "pet name" a couple might have for each other, like "sweet cakes" or "sugar cakes" or "honey buns" - only instead of expressing affection you're making fun of their gigantic moon-eclipsing park-bench-swallowing fat ass

2) The other meaning is obvious if you spell it "bakin' cakes" instead. Now you're saying that they spend all their time feasting on pastries like a whale-shark inhales krill, and this is why the family dog shrieks in terror and leaps off the couch whenever they approach, in desperate fear of suffocating to death in their sweaty skin folds (only to have its dehydrated carcass discovered weeks later when the smell starts to attract buzzards)

*but mostly disparaging