First of all, Mister is totally right; you can't diminish bone structure. On a more personal note, I never had an ass to speak of, and even the lack thereof has gotten smaller on T. My hips have gotten smaller in terms of fat, but I've always had wider hips (though probably not for a chick), and that hasn't changed. They are, however, small enough at this point that no one seems to question my being a dude. This is after over a year on T... and pretty much, the fat redistribution will make the difference between your hips and the rest of your torso fairly minimal. If you're overweight, like I am (though still losing weight), T will put a lot of the fat from your hips into your belly, so instead of having big hips, you'll have a bit of a pot belly unless you're good with the ab exercises.
Not to mention, of course, that quite a few guys have wider than normal hips, and no one questions their being men. If you have a low enough voice, it's typically sufficient for people to peg you as male.
SD