Carbon bikes too stiff for lighter riders ?

"...Is the trend for fitting larger volume tyres to carbon road bikes an admission that the frames are too stiff especially for lighter riders?..."

Large volume tyres are fitted to almost all, not just carbon bikes these days.

Perhaps it's an admission that the role of pneumatic tyres is to provide suspension not to emulate wooden carriage wheels? As we all? know, vehicles with effective suspension roll faster. In other words it's progress driven by science and common sense rather than the strong convictions of some local club "gurus". 

In comparison with suspension travel offered by tyres which can be easily observed and meassured, any "vertical compliance" of the frame is almost insignificant. Oh, and let's don't forget about the seatpost and saddle flex, too. The feel and comfort of a bike is almost entirely dictated by tyre setup, fit and contact points. 

There's a massive, massive difference in feel depending on which bike frame/wheels/tyres etc you go for and I can't help but think you're over generalising here.

You sound like someone who wants a steel or titanium bike but doesn't want the weight penalty (they can be made pretty light if you are willing to spend a bit).

What bike are you on btw? You've tried different tyres and tyre pressures I take it? Tbh I'm not sure 120kg is the limit that manufacturers go to - I've had bikes where it's been 100kg. Don't forget people do load bikes up with accessories and bikepacking bags and so on and so they wouldn't want the hassle of making a too light frame that cracked all the time

I'm far from 65kg, but have two Ti bikes (Litespeed Siena and Vortex), two carbon (Factor 02 and Colnago C50) a steel frame (Gios Torino) and an aluminium / carbon Pinarello Prince all of which are set up similarly Thomson / Deda Stems / carbon bars, Fiz:k seats and I long since came to the conclusion that wheels are massively overlooked when judging ride comfort.  Only the Factor is post 2004. 

I can turn my C50 into a bitch by riding on stiff Shamal wheels on UK roads that are smooth as silk on better road surfaces.  And Mavic Ksyrium wheels seem to shake my vertebra apart. 

That said, these are all decent, well built frames. I suspect that if you were to pit a cheap aluminium frame against a mid range carbon Roubaix with similar components then the Roubaix would be significantly more comfortable.  Higher volume, lower pressure tyres will enhance the comfort of any bike -  if the road surfaces are bad enough (i.e. if you live in the UK).  

Cervelo recently admitted that the old R5 was too stiff (admittedly this admission only came in order to plug the new, less stiff R5...). But I do think you're right - until recently, it was viewed as stiffer = better, and that mantra has driven design of modern bicycles. However, I also think there are signs of that changing, for a few reasons:

Firstly, more real-world data. It's easier than ever to look at power and speed data, and work out what actually works and what doesn't. We don't have to rely on what "feels" fast before - which is good because "feeling" faster doesn't always translate to going faster. 

Secondly, there's (some) recognition that comfort is related to performance - so even if your machine is a few watts slower, if this means the rider is able to keep up for longer, then that will be worth more.

Thirdly, I think it's fair to say that it makes a difference which bits of the system are stiff, and which bits are compliant. I'm no engineer, but it certainly seems to make intuitive sense that a stiff frame with large, supple tyres would be better than a flexible frame with solid tyres, even if the total amount of "flex" in the system is similar. 

I have a carbon framed endurance bike and a ti gravel bike with similar width tyres both running tubeless at 70psi sometimes 65.
The carbon is a little stiffer at the same pressure but there is less than 10 psi worth of comfort difference. I used to have an 853 steel bike which was somewhere between those 2. I'm 72-74kg. 

You're overthinking it tbh. 

I've been thinking about this too over the summer, as I rode around on my 21 year old steel framed Brian Rourke 653 with its lovely curved fork blades. The bike still has a 700x23C on the front on a nice wide 19mm modern rim. Despite this unfashionably narrow tyres - there's a 25 on the back - the Rourke has an absolutely sublime ride quality.

Meanwhile I see all these modern bikes with super rigid straight carbon forks, necessitated we are told because of the forces from disk brakes, and needing fatter and fatter tyres - and fatter rims to match.

Have we lost the plot with these ever stiffer frames and resulting ever fatter - and heavier - tyres?

Well an aluminium bike will also be built for 120kg, so it evens out. I had mountain bikes (on one 456) in ti, steel and carbon and the carbon was vastly superior in comfort while losing little in stiffness. But rather lacking in crash tests, which is why I got the ti. Road bike PX carbon again more comfortable even on thin tyres But aluminium perfectly OK. I always feel good bike fit is the most important followed by tyres.

At around 66kg I can honestly say that the 3 carbon frames I have owned have been much more comfortable than my 4 aluminium frames.  Save your money, dont buy a carbon seatpost, I have had 1 snap, 1 delaminate and one deform around the clamp bolt, the deformed one was a Cinelli, the delaminated one a cheap eBay job and the snapped one was a Selcof.  Think about it, the transmission route for road buzz from road to your bum is through tyres, wheels, frame, seatpin, saddle and shorts/tights padding to your bum, how much difference do you think the seat pin makes in that combination (forget the marketing rubbish)?

I'm not a light rider myself, but I don't hold a lot of faith in frames being that flexible or absorbing road buzz beyond a minimal amount. Tyres are especially good at deforming and absorbing shocks and rough surfaces, so 28mm at around 70 psi is what I use - you may well want to drop your pressures if you're only 65kg (I'm probably around 85kg at the moment).