Unless Little Jude's nickname is Baby Boy or Punkin or something, he as a stillborn baby/small child hasn't had as many opportunities to acquire nicknames as Big Jude has. It's possible that people he's haunted have given him nicknames, sure. But Big Jude has interacted with more people and done more stuff, right? He's got more life, more experiences.

Alternately, you could give them both nicknames. :D