Zen, it isn't just the frame. It's the components also. I have a lugged steel frame with a wheelset intended for no more than 150 lbs. 25 mm tires can make a far more comfy ride (for anyone) but especially so for larger folks. I had a mtb bar that was marked for riders under 160 lbs. It's a tad more complicated than just the frame.

The trickle down theory of engineering is heavily applied to bicycling with the lower cost bikes benefiting.