Small outside, massive rooms

I notice Bethesda used this tatic crafting Dragons Reach in Skyrim, how does he be able to model massive rooms in a small ouside, just like the Tardis, Police phone box outside, gigantic inside?

This is not really modeling related - more, how a game deals with connecting places.

In the case of Skyrim, there is no relation between the outside and inside of a building (seeing as you have a loading screen when entering buildings, look outside windows etc.). The outside is just one huge map with all the assets, and each door is linked to a seperate indoor map that has the interior of this building.