I'm not sure about all of the updates for the GSIII, but the most recent AT&T update (I don't own one, but check the review sites to keep up with stuff) was 82MB and sent out OTA. That was still a 4.0 build though, and not an update to a post 4.0 build.
The latest whole AT&T firmware (I747UCALH1) for the S3 is 1.1gb, OTA updates are differential so the download is certainly far smaller.
iOS6 on iPhone 4 devices (not sure about 4s) was indeed pretty big. It weighed in around 800MB.
The older iPhones didn't get all the new features (for example no 3d maps except on the 4S), maybe it was smaller because of that? I'm pretty sure it was almost 1gb (980mb?).
Indeed, but an update that large? Not very likely.
It's probably like the playstation 3 where there wasn't a differential update system and the whole firmware has to be downloaded every time. Or maybe since those are launch consoles they just shipped them with a very basic firmware leaving them time to implement the remaining features in the months before launch.
The ROMS for the Galaxy Nexus based on Jelly Bean are usually < 300 MB (compressed). Even uncompressed, it's less than < 500 MB usually.
Aren't those basically vanilla ROMs since the Nexus software is handled directly by Google? The official Samsung phones usually come with tons of crapware (same for other manufacturers) and the operators love increasing the crap tenfold.