Changes in mean adult body size may be a universal response to global warming and sometimes lead to conservation concerns. We show that size reductions in sea turtles are now the norm and have another explanation. From 18,707 measurements of nester size (curve carapace length) spanning 30 years from Redang Island (Malaysia), where nearly all nesting individuals have been tagged, we show that the mean size was initially fairly stable and then decreased by 4.0 cm from 100.8 cm in 2005 to 96.8 cm in 2022, which likely translates to a change in mean mass from 120 kg to 105 kg. At the same time, nesting increased from around 300 to 2,000 nests per year. Consistent with this finding of a size reduction in an expanding population, at 27 of 31 sites across the globe where changes in the mean size of nesting sea turtles have been assessed, mean size is decreasing, and the most marked decreases are at sites where population size is increasing most dramatically. Taken together, these focal and global findings suggest that an important driver of size reductions in sea turtles is an influx of small first-time nesters (neophytes) in expanding populations, and hence, size reductions are partially a consequence of successful sea turtle conservation measures and population recoveries. At the same time, the focal observations in Malaysia show that the mean size of neophytes has also been getting smaller over time: from 99.6 cm to 96.8 cm between 2005-2022, likely because of a change in foraging environments. While smaller turtles have lower reproductive output, this negative consequence of decreases in nest size will often be more than offset by increases in nesting numbers that are occurring widely.