the bigger the install size, the more content that will be on disk
AFAIK, actual playable content often takes up relatively little space. Sure, there's the new maps, but by and large those re-use the same textures, skins, and even entire layouts; the rest is just some DB code dictating npc behaviors, quest objectives, etc.
Its entirely novel new objects / creatures / npcs with unique skins, bump maps, etc that take up a lot of space, and Skyrim uses those only very rarely, for very special events. Even then, they tend to re-skin existing models and so on.
I've got about 400 hours into Skyrim, and plan on starting my second character to I can do some questlines I intentionally ignored with the first. There's also the 100 or so hours I put into developing my alchemy website, and god knows how many on forums.
I'd say those 3.8 gigs (plus some mods) managed to hold my interest.