For me, I believe it is a primal female instinct, seeking the biggest "bull" in the herd, so to speak. Size equals dominance, thus ensuring protection and possession. No doubt it is also the reason I seek sexually dominant men as well. A large size implies strength and the ability to make me submit, thus knowing I am safe in his cage.
Physically, if the proportions become too big then it hurts. So, it's a conundrum. My brain and my body need to compromise!
"Man was framed for War, and woman for the pleasure of the Warrior."