Astrobiological gigantism refers to the potential emergence of disproportionately large organisms in extraterrestrial aquatic environments, particularly within the subsurface oceans of water-based moons and exoplanets. This paper examines the evolutionary, ecological, and environmental factors that could drive the development of gigantism beyond Earth, drawing parallels with deep-sea gigantism observed in Earth's oceans. By integrating astrobiology, planetary science, and evolutionary biology, this research aims to establish a framework for understanding how life may evolve under extreme conditions and what implications such discoveries may have for the search for extraterrestrial life. Further, this study evaluates specific celestial bodies—Europa and Enceladus—that may provide suitable conditions for such organisms. The inclusion of these celestial bodies refines our theoretical framework, aiding future astrobiological missions.