I grow onions every year if nothing else I have onions growing. I never get big onions and the other day I remembered something we were in grandma Powell's garden and she was showing daddy her onions. She had racked the dirt away from the heads and they were growing very large. I have decided that is why mine don't get large because the heads are buried in the dirt up to where the green starts growing.
I am going to try to thin out a few and see if the heads grow larger.