Kids are just honest! They say what they think. I love it when it turns out to be something so hilarious.
And it's not only women who are subject to the "big butt" comments. My parents were visiting once when my daughter was about 4. My dad happened to walk into the living room and stop between my daughter and the TV, where she was watching her favorite cartoon show. Obviously frustrated, she said in a loud voice, "Grandpa, would you move! You're as wide as a buffalo!" We laughed at that one for days.