Strange lightmap behavior (huge artifacts)

I use stationary directional light with CSM shadows, after I bake lightning it looks like that (screen), but if I enable Visualize lightmap density it seems that lightmap resolution even too big for this mesh! (Screen 2).
I think it’s because my whole interior in the room is one mesh, but again in other room I have same one-mesh interior with smaller lightmap resolution and it looks almost perfect even in preview bake
What’s the problem?
Excuse my english)