Different players. Miller has displayed the effort your questioning when he was an oversized teen in the younger grades. Some of the HM wins are on the back of Miller putting up huge post contact meters and long minutes. Does that impact his defence, surely but I think your going off too early and on that note a player like Miller may appear physically capable of handling first grade in short stints but to be able to play the way he did in the juniors it would take far more minutes in Flegg and cup to adapt to that standard.
Laullii is a born leader and plays in that fashion. Very Cameron Murray like in the approach to effort.
The more minutes Miller plays the more effort he will be capable of.