The yield strengths of micropillars of four metallic glasses were measured and subsequently analyzed using Weibull statistics. It was found that the strength variation scaled with the occurrence frequency and volume fraction of cast defects resulting from insufficient castability. The current finding is related to the size effect in bulk and micropillar metallic glass specimens. We also offered a theoretical estimate of the critical size of casting flaws in metallic glasses, above which fracture is expected to initiate.