Actually, the slighter and quicker players usually impact more quickly than the stronger types because of the style of game they play. Look at the debate a few years ago about McLean v Tenace. Everybody would say that McLean wouldn't develop because he was already strongly built, while Tenace had more physical development so he would therefor overtake him when he muscled up.
The difference is that Tenace plays a very outside game while McLean is inside. This means that McLean has to take hits and win contested footy inside packs against men who are much stronger. But more development and he is now a very good player because he can compete in the clinches. Tenace, on the other hand, never has to go near another player because he is an outside player. Even as he bulks up the effect this has is very small, as he is not competing one on one.
Hence you will usually find inside ball winners devlop much later than outside runners. Look at Danyle Pearce. How many tackles did he need to break this year?
Hayes seems like an ouside runner with evasiveness. Hopefully this will result in him showing us what he's got earlier rather than later. Mind you, this doesn't take into account a host of other external factors which can affect development.