I have never tried Callanetics to relate to your experience, but I do have a similar butt issue - my mom says I have a J-Lo booty now that I've been using kettlebells for about 5 months.
I was mortified, but as my weight and measurements pretty much remained unchanged, I am embracing it and I think it looks awesome. I definitely have to get used to how clothes fit - I mainly wear skirts and dresses in the summer, so I'm am deathly afraid of trying on my dress slacks once the weather cools off.
Anyway, I frequently use Mandy Ingber's Yogalosophy, and there's a line in there where she is talking about people with bigger butts and she says, "Just dig on what you've got..." So that's what I'm doing.