I used to have a pretty big ass for a guy also. Now , girls I used to hang out with that I havnt seen in a while keep saying to me I have a nice but. Id say , do some cardio 2-3 times a week around 25-30 mins each time with an hearth rate of about 160. Also , do a good leg workout . I recommand , squat , lunges , leg curls , leg extensions and maybe some leg press.
And , dont neglet the most important thing. Have a good diet. I recommend you check the diet section and aim for a cutting diet.
I did a maintain / cutting diet and I had great results reducing my ass dimensions , hope this helps you out.
Remember , you have to do a combination of those things I just mentionned , just doing cardio , or just doing legs or doing both without eating right , wont get you anywhere. So if you dont plan to do those 3 things , avoid yourself the pain , and do none of them.