The polycrystalline nature of perovskite thin films suggests that the nonradiative recombination losses may be grain size dependent. We use measured grain size distributions of methylammonium lead triiodide layers to describe the macroscopic solar cell as composed of multiple single grain size cells operating in parallel. Using a model, we show that the grain size distribution results in spatial dispersion of the local open-circuit voltage (0.9-1.15 V), fill factor (50-82%), and power conversion efficiency (7.5-19%). When the device is held at open-circuit voltage, there is significant current exchange between large and small grains. Smaller grains are a "parasitic shunt"for the larger grains.