Now I'm worried that ESV won't have enough disc space to be really fulfilling.
No reason to worry about that. Morrowind, Oblivion and Fallout 3 all have relatively small sizes for the huge game world they offers, mainly because of the plug-in system that they use. While ME2 which use the Unreal 3 engine store the game data in a rather bloated way and thus make the games rather large even though the gameworld is noticeably smaller.
Anyway, http://www.gamesas.com/index.php?/topic/1102619-official-tes-v-speculation-thread-47/.