This is what happens. It doesn't happen to the other bones, and I don't know how to fix it. I've tried giving the bones different rigify bone types, and I've tried putting more bones in the finger. I've also tried to adjust the position of the bones, but I can't get anything to work.
To fix the scaling, change the names of the vertex groups (so that the rig doesn't affect those groups) like index.L to Index.L. then add a hook modifier to the mesh. Select the rig as the object, the bone you want to control the vertex group as the bone, and the vertex group you want to control as the vertex group.
Do this for every bone and vertex group pair you need to fix, and that will fix the problem.