How to reduce Terrain's polygon's count? To optimize performance

I need a small terrain 200 * 200. I create one. It looks like it has the same polygon's count as 1000 * 1000, so my small terrain just become a lot more detailed in shape.

I've been searching for the way (terrain option) to reduce polygons count to increase performance. And can't find any. Maybe i do not understand something? Or maybe poly count doesn't really matter so it's always the same for all sizes?

I tried to tweak "Detail Resolution" and ""Detail Resolution per patch" and it doesn't change anything. I use wireframe mode to check poly count and it's the same. Help please.

 

pixel error