Don't just look at the tube thicknesses, but look at the butting profiles as well. You'll find that as you go higher up the scale, the gauge thins out, the butts get shorter, and the tapers longer, and/or the thinner gauge gets longer. Some tubes also have a double zone butt profile. I'm not going to do the calculus on this, but I'll say that you just might end up with a lighter tube overall.

631 tube @ 25.4mm x 600mm long has 0.8/.05/.08 wall thickness, and a butt profile of 100/50/300/50/100mm.
725 tube @ 25.4mm x 600mm long has 0.8/.05/.08 wall thickness, and a butt profile of 85/40/330/40/85mm.
853 tube @ 28.6mm x 600mm long has 0.7/.04/.07 wall thickness, and a butt profile of 40/50/350/50/110

Found at - http://www.torchandfile.com/assets/i...ist%202014.pdf

The higher up the scale, the more choice you have as well when it comes to diameter, wall thickness (looks like they are thinning them out more), and butting profiles. This should allow the frame builder to not only build a lighter frame, but a stronger one as well. It also gives them much more control in being able to fine tune the ride quality of the frame.

The other thing about wall thickness and oversize dimensions is suitability. Steel is strong, but it is also heavy. Oversizing is the best way to stiffen a frame tube, but with steel, you can only take this so far before you start to incur a weight penalty, along with walls that are too thin. Nobody wants a heavier bike with fragile tubes. In this respect, it is better to go with a different material if those are your goals. I can have a super stiff track bike built out of aluminum, one that will be much stiffer than a steel bike, and still have it be lighter. Because of Aluminum's density, the walls will still have sufficient thickness to not be too fragile. To try and offer tubes in steel that approach this compromised overlap would be futile, as no one would use them.