To get right down to answering the question of whether it was wrong for you to like the idea of someone not considering you a fat chick.... absolutely positively not!
As women who are on the quest to seek their thinner self, sometimes (or most of the time/all the time) we get too caught up on what we perceive ourselves to be rather than what we really are. And to that, if a comment comes from an unbiased party changes that perception we have of ourselves, then fantastic! because we need that reality check! It's blatantly obvious just by watching that show "How to look good naked" because when he sets that line of women up from small to big and asks his subject to show where she fits in, she'll always pick the spot that is 3 to 4+ sizes bigger than her.
Breathe in the wonderful sensation of being just a little less self-conscious of your size. Because society has quite effectively made even size 0's think of themselves as "fat chicks".