I'm the biggest my current gf of 6 years (and someday wife) has had, because I'm the only one she's had, so I guess I defaulted into being the biggest! I would say that I wouldn't have to be, but it could be different had she previously had bigger.
For the girls in college I slept with, I was each of their biggest. My 1st gf in college, I was her first and she is married now, so odds are I am larger than him, not that I care. I was also my best friend's 1st, so biggest there by default. One girl I slept with when she was a freshman in college previously had a bf in high school and I was bigger than him.
I guess it's nice to know, but it's not something I thought about much.