Since when were sizes 10 and 12 plus sizes?? And quitting in disgust?? This all goes back to the media creating impossible images for women to live up to. Good for the designer for wanting to show that average women can wear his clothes.
I don't think the point is so much that 10 or 12 are actually plus-size. It's that models for plus-size clothes might wear a size 10 or 12, just like models for "regular" size might wear a 0 or double 0 Plus-size models are going to be smaller than their target consumer, just like regular models are going to be smaller than their target consumer.
I know I'm chiming in late but I remember hearing somewhere that plus sized models are usually 10s and 12s because any bigger than that and the clients think the clothes don't look as good. *shrug*