Because a big girls buts had more curves for the fabric to wrap around. And like most men they can't handle all that thickness so, they have to find out how good all that arse really is:)
Big Girls Are Whats Up! Speaking as a big baby girl myself! Girl, I don't thank they ready for this jelly;)
I think it's science,friction, and cloth to As$ ratio,etc. The elastic is stretched sooooooo tight on the big girl, the panties HAVE to 'go somewhere' ! Not so on a lil girl. Ya know?