We studied granular aluminum thin films by means of transport and THz spectroscopy, which allow us to determine the superconducting critical temperature T_c, energy gap Δ(0) and superfluid stiffness J(0). As the grains progressively decouple, the coupling ratio 2Δ(0)/k_BT_c is found to increase above the BCS weak coupling ratio 3.53, which is shown to be in the BCS-BEC crossover region for the high resistivity samples. Surprisingly, the Mattis-Bardeen (MB) formalism describes remarkably well the behavior of the optical conductivity σ_1,s /σ_1,n for all samples up to very high normal state resistivities.