"You look underweight"

It's like "Excuse me? Say what?" Let me rewind a little. I was talking to my dad this morning in the car on the way to school. We got onto the subject of my "eating disorder". He was telling me how my body dismorphic disorder is really messing with my head. My grandma had told him that she found a tape measure in my room. My dad said he knew what was going on. I was just sitting there thinking that he had no idea. He told me he knew I was obsessing over calories and exercising. He said he can tell that I've lost a lot of weight. I told him I don't see any difference. He said that's my bdd. Then we got onto the topic of my thighs. Now I hate my thighs with a passion so this was kinda a touchy subject. He was saying that I'm not really going to lose any fat from my thighs because it's mainly muscle. I said "yeah, well I need to lose fat from my stomach. I know I have fat there." My dad just shook his head. He said, "Your waist is so tiny, I could wrap my hands around it and have my fingers touch." It was my turn to shake my head in disbelief. There is no way, absolutely NO WAY, that I am that skinny. What is he seeing that I don't see. To me, I look the same as I did 10 pounds ago. I want to look skinny. If he says I am but I don't see it, how long will it be til I do? I just want to be pretty. I just want to be thin. Why can't I see what other people see?