Shaquille O'Neil Disease
A horribly debilitating birth defect where a person who is of very large stature has an average sized penis. The problem is that the average sized penis looks tiny compared to the persons huge body. 
Girl: Damn you have a small dick
Guy: Bitch! its 8 1/2 inches long. I'm 6'8" it just looks small.
Girl: Sorry I didn't realize you were afflicted with Shaquille O'Neil Disease.