Question about mannequin head

I recently ordered this mannequin head, problem is, the old fursuit head I was working on (which is the one that was built on a balaclava and has a moving jaw) can not be slipped over all the way and the jaw of the fursuit head rests differently on the mannequin as opposed to my head. The fursuit head fits my head fine, though. I was planning on scrapping it anyway but I was worried about if I tried to build a new head using the mannequin, it would be too big for me.

While the mannequin head may be fitted for adults, I'm not fully one yet, so I'm not sure if this would be a part of the problem.