In rec.crafts.metalworking Josh Fowler wrote:

4130 is no lighter than mild steel, their density's differ by only
.02 g/cc.

Oh I'm fully aware of that... my understanding is that due to the
increased strength of the material, you can get away with using a thinner
wall for the part and thereby decrease weight, hence chromoly's use in
weight critical situations. But I do realize that for a dimensionally
similar component there is no weight savings by using chromoly. Otherwise,
things like bicycle frames wouldn't be made out of tubing with such thin
walls... they'd have regular old thick walls, and have some sort of magical
weight loss associated with them. That, however, is what aluminum is for!