I don’t know, Suzie, Julie, Vicki, are all names of grown women in my family. I know pleny of girls named “Katie” and “Allie” and such.
I think the nickname-sounding names are cuter and more friendly-sounding. “Robert” and “Donald” sound really formal. If I had a name that went into a common nickname, I would probably use it. “Dominic” really has no real nickname other than “Dom”.
Then you have my boyfriend’s name “Rory”, which sounds like a nickname, but it’s not. Those are even less common. It’s that /i/ sound that’s associated with smallness across many languages.