I've been looking online at the Osprey Rook 50 and Rook 65, as my son has just ordered the 65 - rather puzzlingly, they quote a weight of 1.6kg for each. Now whilst I don't believe for a minute that they weigh exactly the same (the weights have obviously been rounded to the nearest 100g), it does emphasise that there is a minimal weight penalty in going for a slightly larger pack.