Energetic protons are emitted from thin foils irradiated by short laser pulses at high intensities. One- and two-dimensional particle-in-cell simulations have been used to study the influence of initial proton position, laser irradiance, and target density profile on this ion acceleration. These simulations bring additional support to the idea that protons are mainly accelerated from the rear side of the target, by electrostatic fields associated with hot electrons escaping into vacuum. The density scale length at the front of the target appears to be the main parameter to increase proton energies when the laser irradiance is fixed.