Fully concur.

But most bike manufacturers cut a lot of corners if they can save a few bucks. And if standardizing chainstay length on all sizes of a model will cut a few bucks they do it. Same with tubing diameter and tubing gauges, most manufacturers will use the same tubing on all sizes within a model, that way the 250 pound big guy has a wet noodle for a frame while a small framed woman has a really stiff frame.

I took another look at the sales brochures from when I bought my two Thorns. On my Sherpa they used two different tubing sets and four different chainstay lengths for different sizes. On my Nomad they had two different tubing sets, but they actually overlapped for two sizes so for those two sizes you could buy the lighter weight tubing or the heavier tubing. My size was one of the two where they overlapped, so I could buy a frame with a capacity of 45kg without rider weight or 62kg capacity. Both of those weight ratings far exceed any load that I will ever want to carry, but I bought the heavier weight tubing to get a stronger bike. For the other sizes, they only had one choice of tubing, lighter for the smaller frames and heavier for the bigger frames. And the Nomad had five different chainstay lengths for their different sizes.

I am sure that there are other manufacturers that also use different tubing on different sizes for off the shelf frames but I really have no idea who they would be. But I am sure you are looking at the more expensive manufacturers for that.