Turtles have been prominent subjects of analyses of sexual size dimorphism (SSD) owing to their mating system and habitat diversity. In prior studies, marine turtles were grouped with non-marine aquatic turtles (NMAT). This is odd because it is well-established that the marine environment imposes a distinct selective milieu on body form of vagile vertebrates, driven by convergent adaptations for energy-efficient propulsion and drag reduction. We generated a comprehensive database of adult marine turtle body size (38,569 observations across all species), which we then used to evaluate both the magnitude of SSD in marine turtles and how it compares to SSD in NMAT. We find that marine turtles are not sexually size dimorphic, whereas NMAT typically exhibit female-biased SSD. We argue that the reason for this difference is the sustained long-distance swimming that characterises marine turtle ecology, which entails significant energetic costs incurred by both sexes. Hence, the ability of either sex to allocate proportionately more to growth than the other is likely constrained, meaning that sexual differences in growth and resultant body size are not possible. Consequently, lumping marine turtles with NMAT dilutes the statistical signature of different kinds of selection on SSD and should be avoided in future studies.