A comparison size between a woman's breasts and their belly/gut/etc. Ideally, men look for women who's breast ratio are larger than the belly.
Likewise, women who have a bigger belly than they have breasts are said to
violate the boob to belly ratio.
"dude, that girl over at the bar is hot!"
"
You're kidding, right? She totally violates
the boob to
belly ratio!..."