Big Mama's are more adept at anal. For one thing, they tend to like it more. Big girls are more likely to take it in the ass. My theory is that it was harder for them to get guys to fuck them. Many gave up their asses in desperation. In order to compete with the skinny girl many Big Mamas sucked cock and were open to try things. We often develop fetishes based on early sexual experiences. If the high school jock wanted to try an ass on for sizie the girl was probably Plus size.