I think if the names are classically popular, even if they are very short they will often have lots of diminutive options, because historically people needed to be able to tell which
William was which! Like look at
John, a simple name if ever there was one, but look at the slew of nicknames that were derived from it.
So examples like
Robert,
William,
Henry etc don't really follow much logic in terms of how short they are. But more modern names, like, I don't know...
Justin,
Brandon,
Gary... Those definitely follow your line of thinking. As far as I've observed, anyway!