The tubes used prior to the availability of air-hardening steels weakened significantly when exposed to the heat of welding. The only way to compensate was to make the tube thicker, therefore heavier. The new classes of steels actually get stronger with the heat, so the tubes can be quite thin. My welded Waterford R-33 has a downtube that is .7MM at the welds. Amazing. And the frame itself, made of S-3 steel weighs 2.9 lbs. Also, with the advent of laser tube mitering, tubes can be cut to fit precisely together, so that a very small weld bead is required.