What does strike you if you look through family archives is what a very small selection of names our ancestors used in any one region, or indeed in the whole of England or Scotland. Despite having large families, they gave most of their children names from a pool of maybe 20 names that occur again and again across the generations. If a child died, they sometimes used the name again for the next child of that sex. The diminutives (Molly, Willie, etc) will have been required to distinguish a child named Margaret or William from their aunt/cousin/father etc, but as Will Fly says, almost all of children were registered using the full, formal version of the name.