The extreme sized breasts are solely for porno and the feature dance circuit; strip clubs love features with 'em b/c patrons love 'em (I guess kinda for shock value). They bring in big bucks in the States (and previously in Canada). If you want to be a big-name feature, you've got to be in porno. And if you want to be a big name in porno, then you better have something to offer (and really large tits usually do the trick!).
They are REALLY bad on the back, as many features who had them attested to (Tiffany Towers, Wendy Whoppers, etc.). Usually the girls have them removed as soon as they stop dancing. They're also really hard to sleep with comfortably.