Body size is a recognised factor impacting mating success of a number of insect species. The black soldier fly, Hermetia illucens (L.) (Diptera: Stratiomyidae), which is mass produced to convert organic waste to protein, exhibits a lekking behaviour necessary for mating. However, it is not known if adult body size impacts mating success and subsequent fertile egg production. In this study, larvae were raised at two densities to produce two size classes of adults (i.e. large and small). Hourly mating observations were recorded in the following studies: (1) homogenous populations of large or small adults; (2) 50% heterogenous populations (equal number of large males with small females and vice versa); and (3) 25% heterogenous (i.e. equal number of large and small adults for both sexes). Adult weight, morphometrics of resulting adults, total number of mating pairs and failed mating attempts, multiple matings, as well as eggs produced and associated hatch rate were recorded for each experiment. Morphometrics and weights in large adults were 21 and 50% greater than small adult males and females by size and weight, respectively. The first experiment of homogenous populations (i.e. large or small) showed no significant differences across other variables measured. However, when populations of different sized adults were mixed equally based on sex (i.e. 50% heterogenous populations), mating success increased 50 to 100% for small males with large females and large males with small females, respectively. Total number of multiple matings increased two to three times. Egg production decreased 15-20% and hatch rate declined approximately 10%. In the 25% heterogenous populations, data were more complex. Number of successful mating pairs across male size was 280% greater than in the homogenous populations. Number of failed mating attempts was also two to three times greater. Multiple matings overall were low (10%) for the homogenous and heterogenous populations. Large males demonstrated two times more aggression in general than small males in the heterogenous than the homogenous populations. Approximately 48 to 343% more eggs were produced in the 25% heterogenous population than large or small homogenous populations, respectively, while hatch rate did not differ among heterogenous and homogenous populations, with an average of 70%. However, the variability in egg hatch was forty times greater than the large homogenous and 40% greater than the small homogenous. While increased egg production is desired, high variability in egg hatch impedes fertile egg production and predictability at an industrial scale.