For the most part that I know of.
This is my guess why. On a pull start engine the lowest part of the engine is the pull starter as well as a rotro starter backing plate. So they make the flywheel a larger OD so in case you want to be able to user a bump starter. Now on a none pull start or none roto start you can mount your engine down lower as the case is the lowest part so to keep the flywheel from being below the chassis or to low the flywheel is a smaller OD. Now all of this has to do with weather you run pull start or none pull start engine mounts.