If you took a saw to the frames, you'd be amazed at how much thicker the tubing's walls are on the Canti frame.
If my memory serves me, your Canti-frame Schwinn is an imported one.... but those old Chicago Schwinns, man, they were made of some really thick-walled stuff. You know it's the truly stout stuff when you take a regular diamond frame "lightweight", strip EVERYthing off of it, and it still feels as heavy as a complete modern roadbike. I rode like three miles with a Varsity frame on my shoulders--no fork, just the frame, the headset cups, MAYBE the BB cups....and it felt like aHerculean task....