This study focused on the populations of Locusta migratoria (Linnaeus, 1758) on Tsushima Island, Japan, which exhibited the largest mean adult body size when compared to other populations from various climatic regions. Certain individuals, designated as “giant locusts,” displayed exceptionally large sizes, with females and males identified when with head widths surpassing 10.5 mm and 8.0 mm, respectively. The maximum weight recorded for a giant locust was 8.9 g, in contrast to other examined females with the maximum weight ranging from 3.1 to 5.5 g. Notably, giant females exhibited the ability to yield larger egg pods and progeny compared to their counterparts. A positive correlation emerged between body size of adult females and the number of ovarioles. To explore the potential heritability of body size, selection experiments were conducted across two generations. A significant impact of selection on adult body size was apparent at LD 12:12h, whereas this effect was not evident at LD 16:8h. Furthermore, crossing experiments showed that body size at hatching closely resembled that of the female parent or demonstrated dimensions intermediary between the two parents, suggesting a complex genetic basis for the observed body size variations. This study provides no evidence of genetic differentiation between the giant locusts and the other locusts on this island.