Reproductive investment (e.g., egg size) is generally critical for the successful establishment of invasive species, with high variability often positively influencing success. Silver carp Hypophthalmichthys molitrix (Valenciennes, 1844) are highly successful invasive fish on a global scale and threaten biodiversity in a wide range of freshwater habitats. However, factors influencing egg size variability in silver carp are not fully understood. We collected drifting silver carp eggs from the Wabash River, Indiana (USA) throughout the 2012 and 2013 spawning seasons to examine egg diameter variation was related to environmental factors and across time. The Wabash River is largely unregulated with few backwater habitats, and the resident silver carp population is relatively new. As a result, the Wabash River offers the opportunity to observe an expanding silver carp population in a river with a comparatively “natural” flow regime. Egg size was positively correlated with both embryo total length and yolk sac volume, indicating larger silver carp eggs contain larger, better-provisioned embryos at a given developmental stage. Eggs collected in 2013 were significantly larger than those collected in 2012 despite a decline in mean length of mature females. Relationships between egg diameter and environmental factors appear complex, with relationships varying between years. Silver carp eggs displayed high variability in diameter, with a mean coefficient of variation of 9.5%. This high variation may reflect a diverse population of spawning females, but it may also constitute a reproductive strategy to facilitate egg transport across habitats and potentially reduce intraspecific competition. Such reproductive plasticity in silver carp and other invasive species is likely to be key for achieving reproductive success in newly invaded ecosystems despite the species’ naivety with novel and unpredictable environmental conditions therein.