As with Oregon, the size of the items (not just the weight) dictates the size of the bag. You can have very light yet still bulky items or you can have bulky but compact items. It varies. A simple weight chart is great (and normally not that realistic) but will only give you a guide.

Let's figure another angle: Each brand of bag AND type of bag within a given brand fit differently with similar weights. Say you put 30lbs in BigBag's Megabag and it felt like you had air, but that same 30lbs in BigBags Semi-MegaBag could feel like you just tried to carry a steam roller up the side of a mountain. The weight is the same, the volume is the same, but the bag will feel different.

If you can possibly do so, get the gear first.