I don't think it really matters what size you start, if you were over weight for a while (for me about the last eight years...), it is still a shock to look in the mirror and see a thin person...even when you know, intellectually, that you have attained your goal. It takes a while for the brain to register that the image in the mirror is thin or thinner or whatever. I still feel most of the time like that pleasantly plump mom I have been for so long. It startles me when I walk past a plate of glass and realize that is me in it. In fact, I had a dream about a week ago that I had dreamed the weight loss...it was sooo vivid that I reached down and felt my hipbones to make sure they were still there!
You will get there...imagining thin when you have never been thin in your adult life would be different, I think, than my experience -- more difficult to imagine a "slim" you. You will get there though, and you will be the thin girl, and my guess is that it will be that much sweeter when it happens.
Brains are odd...