I have terrible vision, ever since I was five years old I have had glasses, and they are very strong. When I take them off and look in the mirror I look a lot skinnier then when I have my glasses on. I was just curious if the glasses are magnifying my body and making me look larger, or if that is acutally the way that I appear to everyone else. This is really stumping me and I'm not quite sure how large I am. Any answers are greatly apreciated :)
Thanks.

Measure yourself with a tape measure and record the results. If your clothes fit well and you are physically comfortable you're probably fine. What do your family and friends say? I can't see you.

Different mirrors give different reflections, just like weight scales being off. Differing lighting can make you look different. Taking your glasses off can alter not only your vision, but also the way you process light. When I want to feel like I look great, there is a mirror I choose, and a mirror I avoid.

Yes this is a possible phenomenon resulting from the optical properties of glasses. A lot of research and trial and error has gone into making glasses distort what we see as little as possible, but there will always be some distortion, which may or may not be noticeable.