A contraction of that ever so commen name...Leopold Old?
I'd imagine Leonardo is more common than Leopold and always has been generally speaking. Also, my (very culturally aware) phone autocorrected Leopold to Leopoldo...is the former a contraction of the latter?
I hardly think Leo diminutive of Leopold is the same kind of thing as Jack diminutive of John, Harry of Henry (it's not btw it's a popular myth, Hari is a Welsh name angliscised to Harry, popular nickname for Henry).