Collider size question

  1. Create a new project
  2. Create Other GameObject -> "Cube"
  3. Create Other GameObject -> "Plane"
  4. Move Plane to -1 in Y direction
  5. Replace the box collider on the Cube with a Mesh collider
  6. Change both colliders to convex
  7. The Cube collider matches the shape of the cube
  8. The Plane collider is orders of magnitude thicker then the plane.

Can anyone tell me why?

The collider size also does not seem to be affected by the Y scale.

EDIT Note: I haven't accepted one of the answers below as I was referring to two meshes both with convex set. It seems it's not possible to over ride the size of a mesh collider on built in objects.

because a collider should have a value greater than zero in all directions. a plane is a plane of vertices with normals and even is not visible from the opposite side but colliders are shapes and their calculation has nothing to do with the mesh. as you understand the plane has no thickness because it's just a series of vertices with their normal in same direction. there is not a real world shape for them in 3D and you can not find anything with only one side available in real world so their collider shape should be a verry thin box like shape.

When you add a collider component to an object with a mesh, the collider auto-sizes to fit the mesh, but that's just a convenience. You can adjust the size of the collider manually in the inspector, in the collider's "size" property - see the Box Collider section of the Unity manual (in the Reference section).

A possible "work around" to my question is to create and resize a cube to match the size of the plane. Of course this add's more vertices/faces but at least the collider will match the shape of the cube even if I make it extremely thin.

Replace the Mesh Collider on the Plane with a Box Collider. It will fit your plane, and you can adjust the Y thickness (I tested 0.001, which still stopped a falling rigidbody).