Has TileAreaDefs.xml something to do with that? And what is the meaning of the contents (Layout tag)? I changed the Layout of SimpleNebulaMedium1 by arbitrarily replacing 0s with 1s, but I cannot see any effect.
Edit 2: Ok, it has something to do with nebula size. The Layout somehow determines the appearance of the nebula, and the width and height determine how many tiles of the map are to be filled with the numbers from the Layout, counting from the lower left edge. Before applying that to the actual map I think the nebula layout is randomly rotated.
How that all works exactly I cannot say, and it looks like you cannot use the complete Layout. With width=16 and height=12 I can generate a map, with width=26 and height=22 I cannot, the game crashes, although the Layout has a size of 31x31. I didn't check other values yet.