I wasn't speaking anything about seat tube angles with my last post. But here's seat tube angle facts.

Seat angles are solely a factor in fit by giving a slightly different range of saddle fore<->aft positions (but seatposts can adjust this range of positions too). Steeper angeles and seatposts with less setback encourage a more aggressive riding position being more over the bottom bracket. So you could say a 72d seat tube is more comfortable than 73d, but that wouldn't be because of stiffness or such. Just the riding position, and it'll probably have higher stack with less reach as well. Seat tube diameter is a factor in frame 'feel' though. (Section modulus)

As for smaller head angles: With the forces in cycling, the range in headtube angles isn't something that's even measurable in frame flex. Even if they were to make a roadie with a 60d headtube, they'd just need to give it a tapered headtube to make up for any unwanted flex. But they wouldn't do this even if they wanted this due to how it would handle! What they do to give a little vibration smoothing is an adjustment in fork rake. Just making the axle less in alignment of the steerer tube.

Around 73 degrees has been road bike angles for seat and head tubes for many years because they've found it to be the best for fit and handling, turning at a comfortable rate for the speeds a bicycle go at, & giving saddle positions that work for femur lengths (fore <-> aft). Smaller frames can have steeper seat tube angles because the wheel would touch the seat tube if it were slacker, and sometimes they make very large frame sizes slightly slacker to accommodate often larger femur lengths of very tall people.