It makes more sense to do an incremental patch - replace files, remove old ones, add new ones, etc. Look at home many phones get updates, then how large the update is, then how much it costs VZW to issue OTA updates per MB. Verizon would literally double their costs to send out updates by making it a full ROM instead of a 300MB patch. Yes, its harder to patch in terms of ensuring the proper files are modified or replaced, but thats just a script they need to work out.
Data charges are high because mobile data has a large operating cost compared to land networks. In an effort to reduce necessary bandwidth requirements and cost, VZW and manufacturers are smart to release patches instead of full replacements.