Rage uses a new rendering approach in which every surface has a unique texture. In a normal game, textures are reused all over the place. The texture compression Rage uses is actually very good, it's just that the data set is massive.
Their raw data set is much, much larger than what they shipped. Carmack has given out figures like 300gb per level.
In order to cull that down to a game that can fit in 24gb, they had to drop a huge amount of detail. The way they did it was quite interesting as well.
They get telemetry on clients playing the game, and then they build a weighted set of textures at each mip that were actually loaded by real clients. Then they take the highest weighted 24GB of them, and that is what they ship.