All steels have pretty much the same density, but because 4130 is stronger, the tubing can be drawn thinner. Most AISI 4130 bicycle tubing is butted or double butted, meaning thicker on the ends where the tubes are welded or brazed, and thinner in the middle. Thinner is lighter, so thinner walled butted 4130 is significantly lighter than mild steel, typically straight gauge (not butted) AISI 1020 or 1030 carbon steel.