Originally Posted by Timmi

I have a problem with that quote from the other forum:
a 753 would have been more flexible, not stiffer.
It would have had more bottom bracket flex, resulting in loss of energy, loss of efficiency.

In the cited example/remedy, the 753 seat stays (and chainstays and fork blades) would have allowed the bike to flex a little more on the cobblestones, and the 531 would made the main triangle more rigid, helping to keep the bottom bracket in place while pedaling hard, sending more of that energy into the chain and less into loading a spring (which also unloads once your pedal is past the point where the return energy can be of any help).

Tubing of diameter X, with wall-thickness Y, and butt profile Z, will have the same stiffness, no matter what steel alloy you are looking at, whether it be 531, 631, 853, 953, 753, 525, 501, Columbus flavors, etc., heat-treated or not. The tubing alloys and heat treatments merely allow you to construct a thinner tube that will not fail (break) - but that will not make it stiffer - rather more flexible.

So I am inclined to believe that the riders were complaining of exactly the opposite problem: instead, 753 would have been too flexible. Of course, that would not make good press for their flagship tubing. Another reason had to be invented, that doesn't make 753 look bad.

We are looking at tubing the wrong way:
we should be looking at rider size (ie: frame size, tubing length), rider strength, and what tubing stiffness best suits that rider. You can go 28.6mm with thicker walls, or if you have more money to spend, a larger diameter with thinner walls to save on weight. Diameter goes a longer way towards increasing tube stiffness than increasing the wall-thickness does. But it also comes with an aerodynamics increased air-drag penalty. Luckily, a small increase in diameter can increase stiffness by 40-50% or so.
The smallest of bikes should be made with the likes of (going with old tubing here, of same 28.6mm diameter) Columbus SL or Reynolds 531Pro, the medium sized bikes starting to use C.Cromor or R.531/725, the larger frames the thicker tubing or new oversized (531, Aelle, 953 if you have the money). That will derive the most benefits for your investment.
There are very large marketing machines - and a whole marketing/manufacturers' ecosystem, with lots of money at stake, that are highly reliant on us focusing on the alloy (and label), and not the frame-size/rider strength/tubing-diameter equation.