Anyone have any theories why there is a distinct lack of heat in the scenes of the ultra-large boob stars?

They rarely give decent head, look bored while being fucked, dodge cum like it is the early 80s, etc...

Just curious what it is about them.

Oh, and this isn't about whether or not you think they are hideously scarred whores or not... I am genuinely curious why their scenes are always so lackluster in comparison to other girls.