This is caused by lightmaps. Ways to solve this:

1. Reduce total surface area of your level. Each square meter of wall/floor/ceiling/etc. needs lightmap space, so reducing surface area will reduce lightmap needs. (For example, if your map leaks and you have it inside a giant box, closing the leak with make a huge difference becuase the giant box can be deleted.)

2. Scale up textures on large surfaces. Lightmap usage is based on texture scale, so if the texture is scaled up on a surface, the polygons will use a smaller amount of lightmap. For example, if you have large rock walls, scaling up can help.

3. Usemore of the textures that don't need lightmaps. For example, sky doesn't need lightmaps, so if you have a room with a ceiling, changing the ceiling to be open sky will reduce the amount of lightmaps needed.