Why is ammonium nitrate so water-soluble?

So why should the lattice enthalpy be low? Both ammonium, $N {H}_{4}^{+}$, and nitrate ion, $N {O}_{3}^{-}$ are compound ions, and thus fairly large as ions go, certainly so compared to single metal cations, and even (comparably) larger anions. The force of electrostatic attraction, which in simple terms is Coulomb's Law between point charges, i.e. an $\frac{1}{r} ^ 2$ law, should be smaller because $r$, the distance between ions, is larger.....and this detracts from the lattice enthalpy of the salt.....