Planes that often fly upside down typically have low camber wings. Some of them have almost symmetrical wings. Those wings have less lift in upright flight than a more highly cambered wing would, so (and for other reasons too) the manufacturers compensate with bigger engines.

http://en.wikipedia.org/wiki/Extra_EA-300

and an engineer's question about wing design for aerobatic flight with some good answers:

http://www.eng-tips.com/viewthread.cfm?qid=183605