What to put in a diaper bag??

Ok, this is going to sound silly. We bought a diaper bag. A really nice big expensive one (got a fantastic deal). But I noticed that they are all really big! What the hell do you put in it?? Lol a few diapers, a couple bottles, wipes, soother, a receiving blanket, maybe a change of clothes.... But that comes no where near filling the bag. What am I missing here? I'm due Nov 20th with our first, so I feel like I'm missing something lol