The Formosan subterranean termite, Coptotermes formosanus Shiraki, is an invasive species that originated in China and has been introduced to Hawaii and the U.S. mainland. Colonies are headed either by a pair of reproductives (simple families) or by varying numbers of inbreeding reproductives (extended families), and therefore have variable degrees of inbreeding. Worker size also varies among colonies of Formosan termites. We tested whether variation in worker size can be explained by the breeding system. Workers were collected from colonies from three geographically separated populations (China, Hawaii, and Louisiana), and body weight and head size were measured. Microsatellite genotyping was used to establish whether colonies were simple or extended families and to determine the heterozygosity of workers and their degree of inbreeding relative to their colony (FIC, sensitive to the number of reproductives). All Chinese colonies contained multiple inbreeding neotenics. In Hawaii, 37% of the colonies were simple families and 63% were extended families, both having considerable degrees of inbreeding. In Louisiana, 57% of the colonies were simple families, which were mostly headed by unrelated pairs, and 43% were extended families. In simple families, size and body weight of workers were not associated with FIC or heterozygosity. In extended families of two populations, both size parameters were negatively correlated with FIC; however, heterozygosity was not associated with worker size in any of the populations. This suggests that the number of reproductives within colonies has a stronger influence on worker size than the individuals’ genetic diversity in Formosan subterranean termite colonies.