Insular snake populations show bimodal size distributions with giants evolving on islands with large prey size and dwarfs on islands with small prey size. However, there is virtually no published study that examined growth trajectories of wild individual snakes to test whether giant and dwarf populations of a species differ in their growth rates and life spans. In order to address this issue, we compared the growth history of individually marked snakes in three insular and one mainland populations of Elaphe quadrivirgata. Our long-term field study, which started in 1982, suggests that gigantic snakes on Tadanae-jima, the Izu islands, take twice as long (30 to 40 years) to attain maximum snout-vent length as snakes from two non-gigantic insular populations. Our study also clearly demonstrated that snake gigantism is achieved by continuous growth throughout ontogeny rather than extensive growth early in their life. Contrary to expectations, young snakes from the gigantic populations grew more slowly than those of the other populations. We discuss potential proximate mechanisms that cause these different growth trajectories, particularly focusing on the exploitation of large sized avian eggs by older aged snakes, which would be a key factor enabling gigantism of the snake.