  The exact solution of Kitaev's spin-$1/2$ honeycomb spin-liquid model has
sparked an intense search for Mott insulators hosting bond-dependent Kitaev
interactions, of which $\mathrm{Na}_{2}\mathrm{IrO}_{3}$ and
$\alpha-\mathrm{RuCl}_{3}$ are prime examples. Subsequently, it has been
proposed that also spin-$1$ and spin-$3/2$ analogs of Kitaev interactions may
occur in materials with strong spin-orbit coupling. As a minimal model to
describe these Kitaev materials, we study the Heisenberg-Kitaev Hamiltonian in
a consistent $1/S$ expansion, with $S$ being the spin size. We present a
comprehensive study of this model in the presence of an external magnetic field
applied along two different directions, [001] and [111], for which an intricate
classical phase diagram has been reported. In both settings, we employ
spin-wave theory in a number of ordered phases to compute phase boundaries at
the next-to-leading order in $1/S$ and show that quantum corrections
substantially modify the classical phase diagram. More broadly, our work
presents a consistent route to investigate the leading quantum corrections in
spin models that break spin-rotational symmetry.
