We have investigated the size distribution of germanium islands deposited onto a Si0.84Ge0.16 buffer layer, by atomic force microscopy. The size distribution was found to be bimodal at 630-740 degrees C and consisted of one group of smaller 'pyramidal' islands with a broad distribution of diameters and heights and another group of larger 'spherical' islands with a narrow distribution of diameters and heights. Both the size of the islands and the critical size for the change-over between the two growth modes are strongly temperature-dependent and decrease with decreasing temperature. To explain the origin of this bimodality, we have conducted experiments where the amount of material and the temperature were varied. We suggest that the bimodal size distribution and the temperature-dependent change-over between the two growth modes can be explained by an interplay between energetics and kinetics.