Reminds me of some erotica I read, it's called the Secret Hot Tub Fatty by itsqwet.
It's a story about a woman who's slim up top butt has absolutely massive thighs and a gigantic ass, so when she sits in a hot tub she fills it up entirely because of how big she is. There's mention of fat but only her bottom half is fat. And of course she sits on a guy and nearly buries him in ass, it's great. I'd love to see more stuff like that.
Although in general I agree, the indirect size comparisons are great and I also wish there was some way to describe it more easily. Inconveniently large ass?
Pic not really related, just contribootin'.