Hm.
Big in whose opinion? I think it's all relative.
I'm a size 12 right now, and I feel pretty chunky. That being said, my BFF is a size 16, and I've never thought she looked THAT big. Bigger than me, maybe, but not my definition of BIG.
But there are a lot of considerations, too. I'm 12, but I don't have an ***. Literally, it's like an ironing board. Whereas my BFF has a ghetto booty, and huge boobs... which shove her up into that size 16 category.
I'd say over size 16 is starting to get big. 12-14 is just a little bit chubby. I see sizes 8-10 as society's version of "normal"... and then anything 6 or below, you're "thin."