Shouldn't have been a wifi issue here, it was less than 5 feet from the router... I did some googling around before updating, one site commented that the update size was "typically around 4-5GB, up to 10+ GB if you were unlucky", so who knows how an Apple device decides which package to download?

Yes, agreed, I know if you do the install via your laptop through Finder or Apple Devices the download is about 15+ GB for iOS / iPadOS. My hotel WiFi was quite fast (100 Mbps) so perhaps it just decided to do a large download. As you say, we have no idea what criteria Apple use!