I've always considered the terms evolved from culture rather than the industry. As such, there tends to be more blurred definitions.
Big engine versions of mid sized cars from multiple brands and multiple manufacturers emerged and eventually got defined as muscle cars.
Ford developed the sports car-like compact Mustang and the name pony car eventually stuck on all the me-toos of the other brands and manufacturers. In this case the pony car name includes both big engines and more economical small engines.
