You are not using WEU I hope... WEU is will known to randomly up file size for no reason on every save.
Terrain is stored as 3 files, which can be viewed as sort of images.
Lets say your map is X by Y squares.
there is a X*4 by Y*4 8bit shadow map
there is a X*4 by Y*4 8bit pathing map
there is a X+1 by Y+1 node map
The resulting files are large, often over a MB for the pathing and shadow maps each. They however are compressed so if the data is easilly predictable they can be compressed greatly in size. An empty shadow map is under 1KB for example. However as you add unique data to it the compression rate drops so the file size will increase.