I suspect two things. First and foremost, patching keeps the size down. Imagine all the people updating, even staged like they do in groups, pulling this stuff down. Not everyone's doing it on WiFi.
Actually, it's not so much load on the 3G/4G network that they are concerned about. Pushing it in stages ensures that their update servers can handle the load at any one time. I remember back in the OG Droid days, one of the updates (Froyo I believe) overloaded the update infrastructure on the initial push, they had to suspend the push for several days while they upgraded their servers.