Are rear triangles made today made of thinner / less substantial materials than in the past? I read this question posted by someone else (details below) but without any response:

One thing I've noticed about newer bikes vs bikes like this [referring to a Specialized Hardrock aluminum frame MTB] from
around 20 years ago is that the rear triangle of the older bikes are
typically a lot beefier.  on some of the newer bikes the rear looks
paper thin, particularly the seat stays.  What is your opinion on that
trend, are the beefier rear triangles from yesteryear better or the
newer, more forgiving modern frames that look like they have a lot
more flex to them?

(From a comment to video 
                
             by 'madcapper6' -- though the comment was tangential to the main subject).
Firstly, is the core observation about rear triangles getting smaller accurate? Assuming its true, what's the reason for it?
This is somewhat of an opposite observation to another question (Why are bike frames getting so much bigger) which was about the embiggening of frame &amp; some components.
There is some validity to the observation.
In aluminum, hydroforming on mainstream bikes only started in the early to mid aughts and got more prevalent and sophisticated from there, eventually to the point where even fairly low end bikes often have heavily manipulated tube shapes that would have been much more expensive to produce, if feasible at all, in the past. Hydroforming is often used now to create thin profiles on chain and seat stays that in the past on a comparable price point bike would have been a chunkier, continuous diameter tube.
In carbon, many companies have veered towards dainty, compliant rear end sections. There was less of this in the earlier days of carbon.
In steel however, there's been some of the opposite effect. Steel stays on many contemporary bikes are much larger diameter than comparable older bikes. In some cases this is because fancier modern alloys beget larger tubes with thinner walls. More commonly though, it's a mix of consumers being visually accustomed to bigger tubes and also the structural requirements of disc brakes.