Who is going to define "overpower"? The general use seems to be if the mfr calls it a .46-.60 size 2C, .70-.91 4c, then displacement beyond those limits is overpowered. But if you want to hover and pull out readily, or you want vertical rolls and snaps, then you power your model accordingly, and it's properly powered. On many models, you can't safely put on a bigger engine without saving weight elsewhere to preserve a decent wing loading; and strengthening the front end. Many times a bigger engine generates stronger vibrations, which also have to be compensated for. If you deal with these concerns appropriately, and you manage your throttle, then 150-200% of the "recommended" displacement is typically workable and often desirable. If on the other hand you want to fly a scale model with scale power, then the mfr's recommendations probably make more sense. If your model shakes itself to pieces, or drops out of the sky when your engine quits, it's overpowered.