Conjecture. You don't know that for a fact. Given Sulu's comment "Their weapons are powerful, Sir, we can't take another hit like that!", I would question the structural integrity of those Federation starships if they buckle just as quickly from 23 century photon torpedoes.
And yet we can SEE the effects of Nero's weapons and we have direct points of comparison to standard photon torpedoes. Their explosive effects VISIBLY correlate very closely with standard photon torpedoes and phaser strikes; a direct hit against the Kelvin's hull produces a ten foot hull breach through which at least one crewmember is blown into vacuum, and yet the explosion itself doesn't kill that officer beforehand. This seems equally true of Enterprise, which experiences blast damage directly at the impact site but suffers no other ill effects otherwise.
So there's nothing really exotic or unusual about Nero's torpedoes except for the fact that they pack MANY different warheads into a single shot, and it's entirely possible that this feature ALONE is what makes them so damaging to deflector shields.
That's because the Narada started shooting at the shuttles.
The shuttles hadn't even started launching until three and a half minutes into the battle. That Kelvin survived long enough to launch them at all is pretty revealing.
And the Kelvin's critical systems were already failing across the board. Face it, she hung in for 5 minutes by virtue of the speed of plot, not the strength of the vessel.
You're not honestly going to try and claim that THE ENTERPRISE, of all ships in Starfleet, isn't similarly equipped with Hero Shields and a Speed of Plot Drive, are you? I think on some level you know good and damn well that if Enterprise had decided to fight it out with the Narada, both the accuracy and the destructive power of Nero's torpedoes would experience a sudden and anomalous decline in effectiveness.
But we're talking IN UNIVERSE, and a direct comparison of Enterprise to Kelvin is worth making. Kelvin lasted long enough against Narada to make a fight of it; there is no room for a conjecture that Enterprise wouldn't have lasted at least as long, especially considering that in the final scene of the film Enterprise is seen to intercept 27 torpedoes in under 12 seconds.
Anyway, you've contradicted yourself numerous times here. On one hand, you claim "one torpedo each oughta do it" to destroy unshielded vessels (warbirds, the first 7 Fed. ships to arrive at Vulcan)
Enough to destroy a
Klingon ship, yes. That's not that impressive; Khan managed shot down
three of them with his portable BFG, so Nero's torpedoes could be dollar-store merculite rockets (maybe even the same kind that Korris used to destroy a Klingon warship in "Heart of Glory") and have the same effect.
The Federation ships probably took 9 or 10 torpedoes each, assuming of course that Nero even bothered to shoot down ALL of them rather than just smash the first three of them with a torpedo barrage and then watch the others slam into the debris field like a starship-sized train wreck.