Are you going to be using it as an every day bag for the baby supplies or just "back up" for the mommy's "super bag"?
My wife and I have twins (now about to start kindergarten), and the amount of gear we needed to haul around for them was ridiculous. Ten pounds of baby's and forty pulse pounds of "support gear"(between a double stroller, two infant carriers, diaper bags, etc.) YMMV
My two cents on it is get BIGGER then what you think you need. There are going to be times that the "super bag" just might get left behind (I never thought it would happen to me, but with a baby crying and other "stuff" happening around you, the wife talking about something and next thing you know you are half way to you destination and no bag

). YMMV
Two or three diapers and a small compact pack of wipes might not do the trick. One of my little ones would pee, and after you changed him he would poop (sometimes two separate time, just so you would change him). So we would sometimes go through two or three diapers in less than an hour, and that was for just one of them.
We kept four diaper bags (i know seems like a lot, but not really), two were the car only bags (back ups, and had less then the two primary's, both the same and we got them from a thrift store for less then $10 for both) diapers, wipes, diaper cream, powdered formula (pre measured and in small bottles (i had a ton of small Nalgene ones 2&4 oz)), at least 4 bottles and nipples(for the back up bags), pacifiers, water bottles, instant hand warmers(to heat the bottles), changes of clothes (you can end up going through more then you think between spiting up and potty accidents), several towels or light blankets (for burping and wrapping up the little one), med kit for the little ones (we kept an ear thermometer(worked best for us) Ibuprofen & Tylenol (make sure to get dosages from you MD for the weight and age of your little one) and nail clippers(they grow fast and are sharp). The other two were the "primary" bags, I would swap them out every other time we went out (that way it was ALWAYS was stocked and ready to go).
It was a hard learn for us at the time. We had several times that we had to stop and buy items while we were out because we did not plain ahead. It was not fun, and made me feel like such an a$$ for not being prepared.
Also keep in mind where we had two and that we had to have twice the supplies and equipment. Having just one is going to be much easier (well to me anyway, your going to pull your hair out of you head

LOL)
It's over whelming at times but so worth it.
CONGRATULATIONS to you and your wife!!!!