My theory on why fat girls give good head;
1) They need to make up for their looks. Since they can't strut their ass like the 124lb 5'5 girls, they just resort to being easy to get with, and they give you head no questions asked. And it is awesome head. So they try and level the playing field here by luring you back with their awesome head.
2) Hot girls can attract any guy with just their ass alone. So sucking a dick? Oh just GROSS...swallow....hell no. They feel they are above this. They don't need to do all that to attract a guy. So giving head....they hardly do. And if they do...its just enough to get you to shut up, and pay attention to the fact you are scrumping a HOTTIE.