Understanding the dynamics of small populations is critical to conserve those species at most risk. Previous work has identified demographic and environmental factors that can mutually reinforce one-another to drive populations rapidly to extinction – a process known as the ‘extinction vortex.’ However, studies investigating robustness to the extinction vortex in relation to life history and ecological traits have been lacking. Here, we assemble a database of 55 vertebrate populations monitored to extirpation and perform three analyses to investigate whether a key fitness-related phenotypic trait – body size – influences the rate at which populations succumb to the extinction vortex. We find evidence that populations of smaller-bodied species deteriorate at a faster rate, suggesting that intrinsic biological traits can alter the susceptibility of species to the extinction vortex, and may serve as a useful feature for prioritizing which populations to invest conservation effort in.