While one can say that object's (rest) energy is proportional to mass, it doesn't mean that all of this energy is always released in whatever process we're thinking about.
In chemical reactions (such as in TNT explosions) or nuclear reactions, only a small fraction of the initial mass is converted into other forms of energy that we call explosion (i.e. heat, radiation, kinetic energy of the products). Most of the mass is retained in the reaction products. E.g. burning hydrogen in an oxygen atmosphere produces a H2O molecule, which is almost as massive as the initial components. It's only the difference in the binding energies before and after the reaction that is released.
For some materials, that difference is larger, which makes them better explosives. In nuclear reactions, the forces involved are much stronger, which means the difference is much higher - but still only a small fraction of the total mass.
In a matter-antimatter annihilation, all of the mass of the material is converted to other forms of energy.