Either the people saying this just think that bigger butts in general are not acceptable since. as Yame said, many dance forms tend to discourage it. However, from my viewpoint, the dancers with the bigger butts look so amazing when they dance. Being more on the leaner side, I have to work much harder to have the same effect so move your booty with the grace and style I'm sure you have.
There's a lot of talk in the bellydance community in general about body style. Much of the GP doesn't mind seeing bigger dancers but many times they do not costume themselves according to their body type and the GP complains about this. So many times, the bigger dancers tend to resent the smaller ones automatically because of this. But the first hafla I attended had a plus sized dancer with a great costume, and my friend's husband said he actually liked the bigger dancers, ESPECIALLY THE ONES WITH THE BIGGER BUTTS!
So go out there and dance your booty (OFF?- uh, no!)!!
I probably shouldn't say this, butt ... I mean, but ... there's the old thing about it not being the equipment, rather it's what you can do with it. I, who am not even a dancer, think that would be applicable in this booty-ful situation.
I read this thread a little while ago - it's relevant to not just my current shape, but pretty much one of my constants in life (I guess whatever happens, my derriere is always there). I thought that my relative difficulty in keeping hip movements under control was mostly due to this, but in the meantime I've learned that my gluteus muscles are quite weak. In all seriousness, I wish I had not automatically overestimated them simply based on appearances and I now think that having relatively well-balanced muscles that can really support and control movement matters the most...