2. Besides the song, it means the size of a womans' breasts in relation to her overall body weight/size. A skinny, flat chested woman needs "more bounce to the ounce," whereas a slightly overweight woman with large breasts has enough bounce. The ideal woman would be someone with a stature similar to that of Lara Croft's (though she's fictional).
boy: on the contrary, those jeans look baggy on you and make you look skinny. looks like someone needs more bounce to the ounce.
girl: yeah right, dumbass