Interesting question! I've had this problem. I'm a "big girl", and about 6-7 years ago, I had to stop riding The Beast because I was just too big to fit in it. At the time, I was 5'9" and about 260lbs. I carried it well, I'll have to say, but I had a big butt. At the time, I was a 42D also. Believe it or not, The Beast was the only ride I could no longer fit in. It was because of that little divider thingy they have between the seats. If that wasn't there, I would have been golden!
The other ride that I was borderline on was Drop Zone. If I was 5lbs heavier, I wouldn't have fit. Also, the other ride that was quite uncomfortable for me was the Monster. I love tha ride, but if you recall, when they "close up" the bottom part over you, there are two metal bars. One is chest level, and one is in your lap. That lower one cut into my belly and I almost couldn't breathe, so I stopped riding that one as well.
That was years ago, and I've lost over 70lbs since then. I went to Kings Island for the first time in 7 years last weekend and I could ride everything! I missed The Beast and now I can fit in with no problem! YAY!!!!