First, the fastest way to draw attention to your butt is to wrap a sweater or sweatshirt around your waist! Even the tiniest butt will look larger if you do that. (With that said, I've been known to do this out of practical reasons - like I heat up and need to shed a layer).
Second, you are exercising. I know it's easy to be self-conscious in form-fitting exercise gear, but the important thing to keep telling yourself is that you are doing the right thing, getting healthier AND trimmer in the process. So who cares what your butt looks like? As long as you are comfy and the clothing allows you free movement, and prevents chafing.
For example, exercise tights - while you may worry they accentuate every bump and bulge - do a highly effective job of preventing chafing from thighs rubbing together, whereas loose-fitting shorts do not. So this is one example of where function is much more important than form!
Trust me on this one... nobody will be looking at your butt. And even if they do, fellow exercisers - and the non-exercisers too- will be applauding you for your efforts to get out there and get your butt moving!!
To answer your question, when I was overweight and first started going to the gym (this was back in my 20s), I'd cover myself in XL tee shirts, which (because I'm short) would hit down well below my butt and cover everything. I did that for a long, long time... it took me years (long after the fat had all melted away) to look in the mirror and truly see myself as the fit person I had become. It's very common experience for the brain to still see fat when the fat has been long gone. So you may think you have a big butt, but the reality is very likely that you see a bigger butt than you really have!
When I gained back 20 lbs a few years ago and started exercising again after a long hiatus, I was self-conscious at first because when I'd walk or run I could feel the fat on my butt jiggling. That was quite annoying! But I learned that form-fitting tights helped minimize the jiggling. The more spandex, the better.
I do think it can help to have an exercise top or jacket that hits at hip bone. Visually cuts your butt in half and makes it look smaller. Even if you can still feel it jiggling.
Otherwise, try accentuating your better parts! For example, if you are traditional pear shaped, dress more form-fitting on top with bright colors that draw the eye up to your upper half.
