In the 1970's, some steel tubing used for bikes was a bit lower in strength than cro-mo, even when sold in double-butted form for use with lugged construction. So, to build a frame that was equal in strength to a four-pound frame made from cro-mo, the frame ended up weighing about one pound more.
But, it was used on lower levels of bikes, with heavier components, and heavier wheels. On a 28 pound bike, it would have been silly for the manufacturer to spent extra money on "fancy" tubing such as Reynolds 531, just to end up with a 27 pound bike.
The REALLY heaving stuff was the steel used on Schwinn Varsities. They ended up weighing about 40 pounds. In comparison to a Varsity, a 28 pound Motobecane was a "feather weight" bike.
I have found when buying older bikes that I can buy a bike with a Reynolds 531 frame for about the same price as a bike with a generic frame. Given the choice, I would go with Reynolds over generic steel...but some good bikes have been made with those heavier types of steel.