Thats why 2 guys doing a fat chick is called a spitroast
Spitroasting can be done on any sized woman. Just search it on any porn site.
It's actually a reference to the position of two guys being serviced by one girl, with each guy taking one end of her (one guy at the head and one guy at the rear).
The position resembles a pig on a spitroast. The size of the woman is irrelevant.
Also, pretty sure porking is just another euphemism for sex. People do use it with different connotations, but for the most part, whether feelings are involved or not (usually, the way I have heard the term, feelings are not involved), porking mostly has to do with very energetic and frenzied sex.