I've discovered most girls with sizeable wabs are happy to let yours truly nestle the old chap between them for the duration. Mind you, 80% of the tarts I see off are chubby slappers who will do just about anything with a little silver-tongued encouragement. Incidentally, it does seem to help this activity if the girl is on the meatier side. The extra blubber serves to make the tits more pendulous; hence more easily squeezed together, maximizing coverage.
I have little frame of reference on the other factors mentioned here. Where I come from, the price of sex is as many drinks as it takes the bint to fall over.