Its a factor of leverage on the metal at the point of the bolt. If you make the tab 2X as wide, you have to make the metal 1.3X to 2X (dependig on the quality of the metal in the casting...) as thick or the torque of the engine will snap the mounting lug right off.

You would be unhappy to add 10% to 25% to the weight of the crankcase.

It is also a matter of making the engines fit in small places.

A wideer mounting flange would require the nose of the arplane to be wider to accomodate the engine. Not a bg deal on this Triplane. (plenty of room there...) But on a plane such as the 4*40... you wouldn't have room for anything larger than a .25.