I just wanna start off saying, I've seen plenty of guys with big rumps, and I've even seen some guys with butts bigger than even most curvy women. It happens. I worked with a guy once who had a huge butt, and he was taller, so it was much more in eyesight all the time. But he also wasn't the snazziest dresser, and I feel like maybe better clothing options would have aided him (khakis are NOT slimming).
Personally, I LOVE a nice ass on a guy. I'm actually a little upset that I've lost some plumpness from my ass since starting my natural transition, and I workout hard with my female friend on our "butt days" at the gym (500lbs on the leg press, holla!) But I also understand just wanting a more normal, male behind, because even smaller man asses can be beautiful.
I'm not sure what exactly you're working with. It's hard to tell from words if someone's actually got that big an area or if they're just overly dysphoric and viewing it as more of a problem than others do. But if you're that worried, definitely start working out and dieting. There are ways to try to target a certain area of your body for fat loss. They sell sauna suits, and if you don't mind doing some form of cardio at your house, you could try buying one and just wearing the pants while you workout. I haven't tried one yet (thought about getting one for the shirt to try to lose some boobs), but I'm sure they'll help you sweat away some fat like a mofo. I've also used Sweet Sweat with some success in the past. Results will be slow, but if you stick with it, it can work (I lost a lot of flabby arm fat over the course of a few months with Sweet Sweat).
Cardio will of course be your friend, as well as consistency. And if you're having trouble kicking off weight loss, try a short-term diet, and then slowly incorporate smarter eating choices in your daily diet. Right now, I'm trying to lose up top, and I've begun on a mostly vegetarian diet for the next week or two. If my sinuses and the weather weren't causing me so much pain currently, I'd be doing cardio 5+ times a week. It's tough to stick with something that shows results so slowly, but it's so rewarding when you notice it finally start to pay off.

And who knows, if you really do have a lot of fat to lose from those areas, and if your current diet isn't all that healthy, you might even notice results pretty quickly just from making a few changes!