Fascinating discussion of dragon stats here - I agree wholeheartedly on hp as square root of mass, being roughly equivalent to cross-sectional area, as the best way to go, while armour is proportional to thickness *if* we're only talking about (mostly 'natural') armour with minimal deflective ability - plate armour is almost entirely reliant on its ability to deflect damage, not absorb it, and real-world tank, ship etc armours are 'face hardened' so they have a very hard, brittle surface which makes small arms fire bounce off, and a softer, 'gooier' but very tough interior to absorb damage from heavier shells. The face-hardening could be considered analogous to AC* while the interior toughness is Hardness or DR in D&D rules.
Edit:*This requires that heavy weapons like ballistae & tank shells be given the ability to ignore a certain amount of armour/natural armour, though. Eg if you think a siege crossbow would be unaffected by full plate & shield, but begin to be slowed by heavier armour, giving it 10 pts of AC-reduction would be reasonable. I've instituted this in my game for ballistae etc although it hasn't come up yet. A heavy ballista might ignore 20 pts, a 100mm depleted uranium APFSDSDU shell 100 pts, etc.
I think the current rules for dragons approximate 'reality' by giving them both natural armour AC boost and DR for the big ones - DR ensures that swarms of arrows or spears have no effect on big dragons, as it should be.
I think modelling armour in RPGs is a lot more complicated than often recognised, hence I haven't voted either way yet. A DR approach is vital for modern games where body armour is all about absorbing & dispersing kinetic energy, whereas AC has an advantage modelling medieval plate mail. Neither approach is sufficient, though - how do you model the ability of a medieval peasant with a dagger to kill a knight by sticking the blade through his visor? DR systems underrate the ability of light weapons vs armoured targets, AC systems arguably overrate them.
It may be that a mixed approach, giving some armour both AC (deflection) and DR (absorption), is the way to go, and I have advocated this previously. I don't generally have a huge problem with the move away from heavy armour in 3e, it always seemed a bit unlikely in earlier editions how all the fighters always wore full plate everywhere! The max DEX caps are too harsh though, a 'half your DEX bonus' approach would make more sense IMO - a trained knight in full plate could be very nearly as dextrous as the man in studded leather, and probably moreso than the WW1 infantryman going into battle with a 50lb pack on his back!