Why _terrainData.SetHeights() resizes the terrain?

Terrain incorrect size

If I were to say:

 _terrainData.size = new Vector3(_size,_height,_size);
 _terrainData.SetHeights(0,0,_heightmap);

My terrain gets very big, around 16 times bigger, only lateral(x and z) direction, not vertically.

I have found my solution, that is to write this instead:

 _terrainData.SetHeights(0,0,_heightmap);
 _terrainData.size = new Vector3(_size,_height,_size);

Basically to setHeights() first, and then change the .size.

But my questions, why?

extra: At first I didn't know the source of the problem, until after lots of googling, I found http://forum.unity3d.com/threads/155768-Unity-terrain-from-script-setting-heightmap-makes-terrain-to-resize-(a-lot!).