I'm a weird case because I think I have quite decent forward growth (I have a relatively shallow cheek slope) but I also have a slightly retruded mandible (TMJ symptoms) and quite a narrow jaw for my face. I also have all four wisdom teeth with minimal crowding, and there is space behind those wisdom teeth. So while I have forward growth, I don't have the big square jaw or strong chin. I was a mouth breather as a kid. I honestly think the link between mouth breathing and forward growth is less clear cut than it's typically promoted on forums like this. It no doubt contributes, but you can get someone like me who mouth breathed his way through the most important years of facial development but still ended up with quite sturdy bone structure in many respects (I have big wide cheekbones as well). 

Recently I read an article about Neanderthal traits in modern humans and I had almost every single one (including the occipital bun on the back of the head). One of those traits was having all four wisdom teeth with space behind them. So perhaps there is a strong genetic element to it as well as developmental.