I can only give my experience.
My 9 lb 15 oz baby had "sticky shoulders" (very mild dystocia). I delivered him on hands and knees and wasn't aware that this was even an issue til the first post-partum visit. So it wasn't bad. The next time I was pregnant, however, there was some concern about that, since the baby was obviously even bigger. But...10 lb 8 oz baby boy came barrelling out without any dystocia at all (I was also on hands and knees at the time).
So, who knows why the smaller baby's shoulders got stuck? But clearly it isn't all about size.
My 9 lb 15 oz baby had "sticky shoulders" (very mild dystocia). I delivered him on hands and knees and wasn't aware that this was even an issue til the first post-partum visit. So it wasn't bad. The next time I was pregnant, however, there was some concern about that, since the baby was obviously even bigger. But...10 lb 8 oz baby boy came barrelling out without any dystocia at all (I was also on hands and knees at the time).
So, who knows why the smaller baby's shoulders got stuck? But clearly it isn't all about size.