I'm sorry, I have to kill this one... space requirements wouldn't increase THAT much with both the Zerg and Protoss campaigns included, it's called Shared Resources. All the textures, all the models (with the exception of a few map models), are all included in the base game already. So all that is needed for the Zerg/Protoss campaigns is simply the campaign files and the maps. Some voice-over too, I'm sure. All in all I'd say each expansion will probably add less than 1Gb of data to the game, and that's a high estimate.
There is NO reason to think each expansion would duplicate all the disk space requirements of the original game, and if it does, it's because they Blizzard had a brain fart and didn't think to make the installer check and see if the base files were there and just have it install the needed files for the expansion.
And I dunno if they've released this info yet, but I fully expect the next two games to be Expansions, where you guys are talking about them as if they're gonna be SC3 and SC4. I'll be surprised if they're even stand-alone.
I actually think 1GB is undershooting it.
29 (give or take some) new missions, with both in-game rendered cinematics and pre-rendered HD cinematcs along with new voice acting. Not to mention the fact that they might add/change existing resources (ie: WoW Expansions).
If blizzard is delivering the content like this, its because they want to maximize the players experience with the story line. I, for an example would prefer to have to wait 18 months between certain segments of the game to get a high quality, enjoyable experience then have 10 missions of rushed, compacted, jumbled up crap.