It is argued that the growth morphology of ultrathin metal films should fluctuate as a function of film thickness due to the quantum size effect. To verify this, the specularly reflected intensity of x-rays, electrons and He atoms has been measured during the growth of a thin Pb layer on top of an substrate at 100 K. Curiously enough, the expected variation is observed in the He atom and electron scattering data, but not in the x-ray reflectivity. Our explanation is that the differences in the heat of formation for successive atomic layers have a strong effect on the step density, but not on the occupancy of the layers at the low growth temperatures necessary to obtain layer-by-layer growth. This is backed up by the results of a Monte Carlo simulation.