I don't see any problem, it's only 15% surface area overhang. It's not like you're using a 6.5" pad on a 3" BP, that may fly off.
Back-plate for car polishing isn't as critical compare to other types of sanding/polishing work such as stone, wood, or other hard material. Polishing car, you don't put a whole lot of pressure on it anyways, you just let the compound and the surface of the pad work it. So, overhang is acceptable.