The update is 4.2.2 version V50010b, so not KitKat - just a tweak to the original ROM. I can't explain the huge size other than to assume the download was for the entire ROM rather than just the updates. Maybe the OTA update is just as effective, but more data friendly. I suggest going for OTA if possible.
The only benefit I've noticed is that the terribly uneven lighting at lowest brightness settings has vanished. It's a nice benefit to have. I have no idea what else might have been fixed/improved.