I don't consider this an error. That seems a reasonable point at which to allow 'massive' sized weapons on a ship. If there isn't a class at exactly that size right now, so what? A small change to VehicleSize.txt and there would be one. Actually, in my personal mod the Dreadnought is 1250kt so it qualifies for massive mounts. (Battleship is 1000kt and all the rest slightly larger, too.)
The only 'error' I can think of in the compenhancement fle is that there's no way to give SHIELDS a mount.
If we could have scaled shield generators in the same way we have weapons for larger vehicles the system would be perfect.