I don't watch muscle car shows anymore because I'm tired of seeing the label applied to cars that aren't muscle cars.
Muscle cars started out as full-size sedans that were upgraded or modified with performance parts to give them "muscle". Cars like Corvettes were never muscle cars, they were sports cars (small, two-seat, no back seat). The Mustangs started the long front deck/short rear deck compact car movement, and cars that mimicked the Mustang (Camaros, Firebirds, our E-bodies, etc.) were classified as pony cars.
Move ahead a few years, and the sub-compacts have become compacts and now mid-sized cars, compacts have become full size cars, and pony and sports cars are lobbing on to the muscle car name.