The Boobs/Ass debate is a microcosm of white traits and preferences versus nonwhite traits and preferences.
Ultimately, the debate is a choice between quantity of children versus quality of children. In that regard, it can extend further into the debate around things such as time preference and sexual crime rates by race (as a rapist is concerned about breeding writ large, not so much child rearing)
BIGGER ASS
A larger ass requires a wider frame, and therefore, naturally, wider hips. Evolutionarily, this allows for safely having more children overall
BIGGER TITS
This leads to an average increase in mammary glands, and, by extension, increases lactation output. As mother's milk is the healthiest, most important food source for infants to grow properly, an increase in these nutrients lead to stronger offspring in spite of lower numbers.
As you can see in the whitest US states and Europe/Far Asia, the preference for breasts over ass is an obvious choice. Those of caliber will rear few children, but ones of strength and character.
Conversely, the nonwhite world selects for the Ass, because raw numeric spawn output - not the long term societal impact of that spawn - is what matters.
Breast selectors have a "Predator" breeding strategy.
Ass selectors have a "Prey" breeding strategy.