Si nanowires were grown by a vapor-liquid-solid mechanism using indium catalyst particles with sizes between 10 and 200 nm. Transmission electron microscopy analysis indicates that the growth morphology of the nanostructures changes drastically when the particle size is only a few tens of nanometers. The larger wires are monocrystalline, growing with 111 orientation and limited tapering. The simultaneously grown smaller wires appear strongly tapered, due to an accumulation of amorphous Si on the sidewalls, and present a poor crystalline quality. A model based on the solubility of Si in the In-Si eutectic is proposed to account for the size-dependent change of morphology.