Changes in glacier terminus position have been implicated as one of the primary drivers of the rapid changes in glacier dynamics observed across the globe in the last two decades. Iceberg calving exerts a critical control on the terminus position of the vast majority of marine-terminating glaciers, yet calving is relatively poorly understood due to the inherent difficulties in collecting observations of a stochastic process in a dangerous setting. Time-lapse camera and satellite observations suggest that the style of iceberg calving can vary tremendously in both space and time depending on the physical properties of the terminus, ranging from the detachment of giant tabular icebergs every few decades from Antarctic’s floating ice shelves to the growlers produced nearly daily from serac topples along Alaska’s coast. Here we extract quantitative metrics on the relative importance of calving driven by branching and uncorrelated fractures through application of fragmentation theory to iceberg size distributions extracted from high-resolution digital elevation models for 17 fjords around Greenland. We find that iceberg size distributions typically deviate from the widely-assumed power-law form for icebergs with surface areas >0.05 km^2, with fewer icebergs than predicted by the power-law for larger sizes. Icebergs larger than ~0.1 km^2 primarily calve as the result of full-thickness penetration of uncorrelated fractures (i.e., as tabular icebergs). Although the dataset is temporally sparse for the majority of the study sites, the data suggest that iceberg formation via branching fractures reaches a seasonal peak in summer, when icebergs up to ~0.1 km^2 follow power-law distributions. These data provide a novel means to assess the accuracy of iceberg calving models and potentially to constrain the physical characteristics of termini susceptible to the marine ice cliff instability mechanism.