The bomb must be heavy enough to generate the theoretical amounts of kinetic energy required to penetrate deep underground.
The nose and casing must be able to withstand the tremendous amount of friction, pressure and resulting heat experienced while burrowing under ground. By the time it explodes, much of the casing will have been scoured/melted off.
As for the 5,300 lbs of explosives, modern explosives are capable of burn rates as high as 30K ft/sec. Explosive force with any explosive does not increase linearly with quantity of explosive. There comes a point of diminishing return where each additional pound of explosive yields less pressure.
I wouldn't be surprised if something other than TNT is used in the bomb.
I read somewhere that a common bunker buster used (forget the madel) only has something like 250kgs of explosives.
Some of the “bombs” the brits used in Iraq are pure concrete in a hardened shell.(and lazer guided as well) No explosives at all. They are called low collateral damage bombs. Because they are dropped from so high up, they still make a big “bang” when they hit.
Our first bunker busters were built at Eglin AFB and made from surplus cannon barrels.