"Dairy Dukes" refers to the short shorts, originally made famous by "Daisy Duke" from The Dukes of Hazard, that are worn by women of a much grander size...with a much larger booty.
I don't think it's those "Dairy Dukes" that make her butt look big...I think it's her butt.
Some things should be left to the imagination-that fat ass hanging out of those "Dairy Dukes" over there is one of those things.