I think in general that smaller stock is easier to heat treat. No other explanation for success in my hands! Most tools, chisels etc I make using 3/16" round stock.
It can be fun using scrap metal, found steel etc to make stuff, but a fella has to be willing to risk wasting some time if making tools and springs. Pot hooks, decorative stuff, tent stakes, picket pins, flint whappers, etc is a great place to use found iron/steel. I once found some round stock that threw sparks on a grinder like plain high carbon steel, got plenty hard in a water quench, but was useless for firesteels. No sparks; maybe it was alloyed with something for toughness. I don't know enough for anything but getting into trouble. Making mainsprings and frizzen springs requires a lot of planning and skill for me and I'd hate to make a beauty and have it fail one way or the other (bend, or snap). The work going into a knife is plain scary if it would not take or hold a good edge, or chipped easily, etc.