Size fetish stuff. This scene did it to me. I was at the theater with my babysitter and she was this tiny thing. 5'0" at best. I had just gotten taller than her but I was still only 5'4" or so. She was 22 and a daughter of my mom's friend. She looked after me a lot that summer (I was like 12). We went to see Ant Man 2 and this scene came on. I was already rock hard in the theater. She was so hot and so huge compared to him. I was just starting to FINALLY be taller than some of the women I knew and this scene reversed it. But I think it was cemented when she said "I should do that to you. You'd behave so much better like this."
Now I'm 6'2 but I only jerk off to this kind of thing.