The larger the hip the easier it is to give birth but the less efficient the gait becomes for a bipedal animal, since at each step the projection of the center of mass on the ground is horizontally farther from the foot.
That's the reason why women swing their hips much more than men as they walk. While it looks sexy it is not very evolutionary advantageous if a saber-toothed cat is chasing you.

Birds and bipedal reptiles don't have that problem though, since their birth canal does not pass between the hip bones, thus they can get away with horizontally small hips and still be able to lay eggs which in some species can be as large as one fifth of the their body.