We report the efficient generation of high-gain parametric down-conversion, including pump depletion, with pump powers as low as 100 µW (energies 0.1 µJ/pulse) and conversion efficiencies up to 33%. In our simple configuration, the pump beam is tightly focused into a bulk periodically poled lithium niobate crystal placed in free space. We also observe a change in the photon number statistics for both the pump and down-converted beams as the pump power increases to reach the depleted pump regime. The experimental results are a clear signature of the interplay between the pump and the down-converted beams in highly efficient parametric down-conversion sources.