How to implement ground and not expanded polygon with Primitive API

Hello, everyone,
I am trying to implement ground polygons with Primitive API. When I added polygons in GroundPrimitive, polygons were not well displayed. Showed in the picture below, red polygon was expanded to green area. And I tried Primitive not GroundPrimitive, it was not a good ground effect with twinkling effect sometimes…

My question is how can I implement ground and not expanded polygons with Primitive API?
Looking foward to a reply.

Best wishes!