Is there any way to get rid of these gaps? I use meshes to paint polygon colliders, so there is no sprite that I can scale up/down. It seems as if the physics engine is not exact.
See images below:

I’ve had this problem before and I think it is a value in edit>physics2D
I remember there being a gap because it prevents clipping but try messing around with the values until there is no gap. Alternatively , you could make the collider smaller in scale for the rocks.
I solved it by making a script that scales everything by 10.
It also works if you put the game objects in a GameObject with scale 10, and then set scale to 1 on all your objects.
It is a problem with Physics2D and Box2D, so it is not solvable in any other way.