The generous bottoms of a woman are more sexually arousing to many men, because large bottoms indicate a higher level of Estrogen and possibly a high rate of Fertility. The term comes from the act of sex between a man and a woman blessed with generous bottoms, in a postion known as Doggystyle.
In this sexual position, the well-endowed woman is at an advantage, as her large buttocks act as a soft, yielding barrier between her pelvis and the thrusting pelvis of her mate, making the act very comfortable for both parties. Moreover, in this postion, the man is able to see the motion of his mate's buttocks as he thrusts, and this display arouses the male even further.
Note that a number of males find large buttocks, and by extension, voluptuous females unappealing, but by and large,a curvaceous woman usually attracts more attention than a thin and bony woman.
'So why did you sleep with her? is it 'cos she's got a cushion for the pushin???'