To be fully honest - I can relate to this. During my college years, I ate a full-size burger, potato fries, and soda every day for lunch for ~2 years. I never got fat and retained a healthy BMI, never crossing 200lbs despite being a 6'2" male - even though I almost never exercised and spent almost all day sitting working in the Math Lab. I did skip breakfast regularly so maybe that had something to do with it. My family on seeing the portion size (particularly the potato fries - think 3x a McDonalds large fries) thought it must be my brain consuming those calories or something.