In vivo measurement of blood flow in the retina has been made possible with the advent of Fourier domain optical coherence tomography (OCT). Doppler OCT has seen many advances in recent years in algorithms used for quantifying blood flow. We compare the relative retinal blood flow estimates as measured by the standard phase-resolved (PR) algorithm and the more recent moving-scatterer- sensitive (MSS) algorithm as a function of vessel size. We find that the PR-to-MSS flow ratio significantly decreases with decreasing vessel diameter. We also develop a simulation to approximate the scattering from blood cells in tissue and compare the relative blood flow estimates. The flow ratio measured with simulation closely matches that found in vivo. Our simulation predicts that whereas PR underestimates the flow, MSS overestimates it. Our simulation may help to correct for algorithm bias in in vivo retinal flow estimates.