Is bmi somewhat short on A319s? I am surprised that the CGN-based aircraft is an A320. In the past, Lufthansa often used Avro RJ85 or CRJ700s on the route. The largest aircraft I remember as a regular on the route was a Boeing 737-500.
The largest aircraft Lufthansa currently uses for non-hub flying is the Boeing 737-300 (there are a few exceptions and they will begin to deploy a few of their own A320s to non-hub routes next year).