That could also be possible, but it's more than likely a completely separate model and when he gets summoned, the model basics load, but it can't load all the way because the model isn't there, so it just shrinks to nothingness (unloading). But I could be wrong. If what you said is true, shrinking the entire model down to a correct size would make it work.I'm not sure about this.
When I was messing around with a homies mod, I had Zinyak call in as a homie and he had a head, but it quickly shrunk down into his body.
that makes me think his head is the same model, but perhaps it's lowering down to be normal player head height, rather than where his head should actually be since he's way tall.
Edit: Also, did your Homies Mod you have replace a homie to get Zinyak or add a whole new homie?