When my two were in diapers, we made a grocery run for diapers. Diapers aren't any more expensive in Orlando than at home. Kids clothes are tiny and don't take much room. The kids were long off the bottle by the first trip.
When I travel with only adults, we often go into the park carrying only our room key and drivers license. I used to not even worry about rain. Now we go with more - jackets for the kids, water, juice, fruit snacks, wipes. Children have turned me into a pack mule.
And we do end up with more than one large suitcase. My husband brings a bag - because he likes to and as long as we need to check luggage, he might as well. I can fit it in mine, but its easier. The kids each "pack" a carryon - stuffed animals, I think - I'm afraid to look. I'm going to have to watch my son next trip though - his kindergarten bookbag had nearly the entire contents of my junk drawer "just in case." When you are five or six, you never know when a really pretty rock might come in handy. My husband hauls his laptop for work. I bring a carryon for the plane with stuff for the kids and books for me.
We don't find we are missing much. I've bought sun lotion, but because I forgot it, not because it wouldn't fit. I bring a little bottle of Advil for the grown ups, a few cold pills, some children's Motrin and a few children's cold tablets - but that takes up almost no room. Travel bottle of shampoo - we don't bother with anything but shampoo on vacation - no hair gel or hairspray gets packed, no curling irons.
We did bring a lot more than that to cruise. With WDW we throw on shorts (we wear the same pair for two days) and a tshirt and go - so for four people we have 12-15 pairs of shorts (wear jeans on the plane), 28 tshirts, 4 sets of "jammies", 28 pairs of underwear, four swimsuits, a bunch of socks, sandals, wear tennis shoes on the plane. With the cruise we had at least two outfits for every day, three pairs of shoes, and needed a few swimsuits. Now that we have a washer in the room, we probably will be able to pack even less!
Here is a question for you, do you use everything you bring? For me, the off chance I need to buy an expensive bottle of Pepto Bismol in the gift shop is worth not having to drag more through the airport than I need.