  We address the crystallization of monodisperse hard spheres in terms of the
properties of finite- size crystalline clusters. By means of large scale
event-driven Molecular Dynamics simulations, we study systems at different
packing fractions {\phi} ranging from weakly supersaturated state points to
glassy ones, covering different nucleation regimes. We find that such regimes
also result in different properties of the crystalline clusters: compact
clusters are formed in the classical-nucleation-theory regime ({\phi} \leq
0.54), while a crossover to fractal, ramified clusters is encountered upon
increasing packing fraction ({\phi} \geq 0.56), where nucleation is more
spinodal-like. We draw an analogy between macroscopic crystallization of our
clusters and percolation of attractive systems to provide ideas on how the
packing fraction influences the final structure of the macroscopic crystals. In
our previous work (Phys. Rev. Lett., 106, 215701, 2011), we have demonstrated
how crystallization from a glass (at {\phi} > 0.58) happens via a gradual
(many-step) mechanism: in this paper we show how the mechanism of gradual
growth seems to hold also in super-saturated systems just above freezing
showing that static properties of clusters are not much affected by dynamics.
