The design criteria is different, so that's not a completely fair comparison. A Pugsley was never intended to be ultra light and a carbon wonder bike was most likely never intended to carry frame bags for touring.
The real world different is less, probably a pound or two, between different materials when built for the same task. IOW, you can built extremely light steel bikes where there isn't much weight penalty but it wouldn't be any more appropriate for job than a carbon race bike.
For example, I ride two steel hard tails, a Gunnar Rock Hound 29 (the classic one, standard non-taper headset, 9mm dropouts) and a REEB Dikyelous (modern style, tapered HS, thru axles, dropper compatible, etc.). The Gunnar is a great bike, love it like crazy, but hanging any more than a half frame bag is just too much for it. But the nekkid frame is more than 2 lbs (I want to say almost 3 lbs) lighter, too. The tube walls are thinner, the tube diameters are universally smaller, the expected level of abuse each will tolerate is pretty clear.