Sam, good for you and your ever-so perky buns! LOL!
It's been years since I did a Firm rotation, but I experienced bulking of the buns and thighs doing leg presses. Albeit, your diet has a HUGH reflection on that, and at that time, I was not eating clean. I think it matters too what your body type is. I tend to gain muscle easily and when you have a layer of fluff on top of that new muscle, LOL, you look bigger.
To this day, I'm afraid of doing high box climbs because of that, but things might be different this time around, since I'm eating cleaner. Sam, you're lucky your knees aren't bothered by the climbs; those bother mine more moreso than dips.
So, kitkat, you might just need to do them for a time to see how things turn out. If you're wanting to firm up and get a rounder behind, I definitely think they would be something to consider.
Everyone is different, so we're all going to have different outcomes because of all the stipulations, but I guess you won't know until you try something if it'll work for you!
Good luck, and let us know what you decide!