Over propping a four-stroke glow engine can cause the engine to over heat. Over heating causes the "timing" to advance. If the timing advances too far, the engine will respond by over heating even more. You can see where this is going.

At some point, the timing can become so advanced that the engine will backfire. This is really hard on the connecting rod and crankpin. Enough of this treatment can cause a catastrophic failure.

A way to compensate for overloading an engine is to richen the fuel mixture. This may, or may not alleviate the over heating problem sufficiently.

If you are extremely lucky, you may find a point where the engine does not over heat sufficiently to cause damage and it still may produce enough power to fly the model. Many times you will not be this lucky.

The answer is to use a larger displacement/more powerful engine, or, utilize an engine that is designed to turn the appropriate size propeller at a usable rpm (RCV). They sound the same, don't they? They are not.