A question about MeshCollider and GameObject Scale

I have a SphereGameObject which scale is (1,1,1) and use MeshColldier to do Physical detection,
I use 10 spheres with RigidBody and scale is (1,1,1) inside the SphereGameObject and make them move Freely,
but when I Change all of them scale to (0.5,0.5,0.5) the fps will drastically falling in playmode.

It looks like though you changed the size of all objects, but the range of physical detection has not changed.
So what the reason that the fps will drastically falling in playmode when I I Change all of them scale to (0.5,0.5,0.5).