In my personal experience, there is absolutely no connection whatsoever between a girl's figure/size and their vaginal capacity. I've been with big girls, skinny girls, tall girls and short girls, and, much like men and their penis sizes, there appears to be no correlation between their body size/shape and the size of their vaginas.
Although, interestingly, I have discovered that the closer a girl's knees are to her chest, the less cock she can take... So for those of you who either want to bruise her cervix, or are looking to make yourselves 'feel' bigger from her point of view, just get her knees up.