What's the difference between standard and large chain stays, and what IS a chain stay anyway?

This illustration shows the components of a bike frame, including the chain stay.

This is a photo of a bike with standard chain stays....about 1/2 inch in diameter. (no more than 3/4 inch)

This is a photo of a bike with large chain stays... 3/4" to 1-1/8" (This bike is a Schwinn Stardust. The Aerostar and similar bikes also have large chain stays.)