Mesh-collider accuracy changes depending on scale

So I am adding a PhysicsShapeAuthoring component to my meshes, and setting the type to mesh since I need 100% accurate collisions for a simulation. I then set the manually set the mesh variable (same mesh as is being rendered). The results are... incomplete. It's not turning the mesh 1:1 into the collider...

Here you can see a wheel and a tyre:

As you can see only a thin slice of the left mesh is incorporated into the collider, and rather random triangles are incorporporated on the right.

If I scale the meshes up by a factor of about 5-10 the generated colliders are perfect... but is there a better way of fixing the problem?