Programmatic Terrain Data Incorrect Values

I'm instantiating a Terrain object with Terrain.CreateTerrainGameObject(); quite literally:

Code (csharp):

1.  
2. TerrainData td = new TerrainData();
3. td.size = new Vector3(129, 100, 129);
4. td.heightmapResolution = 129;
5. GameObject t = TerrainCreateTerrainGameObject(td);
6.  

And in the inspector the Terrain Width and Terrain Height both display: 516 (129*4).

Why is are these values getting multiplied by 4? Furthermore, how do I set them such that they don't get changed?

 

*sigh*, As it turns out, if I set the resolution first, and THEN the size, the size will stick as desired. I sure wish this had been documented somewhere