specific power is more complicated than just bigger=better. assuming two engines of different displacements but same build quality are both tuned to make as much power as they possibly can without exploding immediately, the engine with the larger displacement will likely make more torque, which means more power at a given RPM. that's especially true if only the bore is changed, and not the stroke. but then, a lot of racers destroke their engines, which makes the displacement considerably smaller, but greatly increases the RPM potential, meaning it'll still make more peak power, but at the cost of torque.
there are a few guys boring out China girl jugs and sleeving them, usually doing 50mm bore which makes the displacement 78cc. I've heard of people going up to 52mm, which is good for 84cc, but from what I hear that requires a good bit of additional modification. Mark Clyde Stewart I think was selling 50mm bore jugs made by Bert Rod. Bert knows what he's doing, but he's the kind of tuner who keeps secrets, and since his stuff is out of my price range I haven't been able to get my hands on it to reverse engineer it.