Board Thread:Manga/@comment-32563139-20170718093831/@comment-1327106-20170718170126

I don't think the size is random.

Rod Reiss also talks about making Historia the strongest Titan, so I think there's a way to specify certain attributes. It might involve adjusting the mixture during creation.

I don't think it's the amount though, because Rod doesn't get injected so he probably got a lot less than intended and yet he still turned out huge when he licked up the leftovers intended for Historia.