The rear mount should be central in the aperture in the bulkhead and should have a clearance all around. if it touches anywhere you will get bumps and vibrations back through the bodywork. This is perhaps why you think that it has shrunk?
The engine sits on two sprung side mounts and all that the rear mount does is stops the engine jumping up and off the springs if you drive it too savagely!! If you replace the volute sprung side mounts with metallastic mounts which have top fixings as well as bottom, then you can throw the rear mount away! This mod also tends to smooth things out when changing gear, pulling away, etc.