How to tone up my stomach and elongate my torso?

So some may say i'm "fat", but I finally love my body. I've gained about 15 lbs but I'm happy with it. I was wayy to small and it just didn't look right on me. Now I am about 145 and 5'4. I love my butt, thighs, and legs, but I need to tone up my stomach. I LOVE being thick, I look so much better with meat on my bones. Although I don't want to get any bigger, I just want to tone up. Is pilatese a good exercise for this? I don't want to do anything with weight because I get muscular pretty easy and that is the LAST thing I want I already have somewhat broad shoulders because I did gymnastic since I was a baby up to 15. BTW i'm 21 now.