Over 3 gigs which is pretty insane. The whole game is worth 5 gigs before.
I'm not too surprised a High Res texture pack is 3GB. Thing is, most of the space taken up by a 3D game are usually the textures. Each texture is basically a bitmap image, and the higher-res the texture, the more pixels per texture. The game has what - hundreds if not thousands of textures?
I believe texture files are basically square or rectangular in shape (when rendered in the game, they might be almost any shape, but the actual file will be approximately rectangular, but have regions of "transparent" pixels which never get rendered by the game engine).
In general what this means is that when you double the resolution, you quadruple the size - if your resolution started off as 300x200 = 60,000 pixels, and you want to double it, you basically create a files that is 600x400 pixels = 240,000 pixels.