JR of all the factors you cited I would put the amount of direct sun at the top as likely cause of size differences. I am in the trees here and find it hard to rotate my garlic into equally sunny spots - they just aren't. And in the years I've been growing here, it's consistently the case that more sun produces bigger bulbs.
In planting depth I have seen the opposite at the farm - that the shallower planted bulbs were smaller. Not sure if there were soil differences as well in that row, but wherever the cloves were planted deeper (about 4 inches deep) they produced the biggest bulbs.
If Henry was around I bet he could tell you what kind of garlic that is. MeganP might know. It could be a rocambole but iirc the creoles also have fewer and larger bulbils like that. And turbans too?
