Age and size at sexual maturity are major life-history traits that affect growth rate and reproductive output. The differences in these life-history traits can be better assessed in syntopic populations of related species, where environmental variability in time and space is removed. The Spadefoot Toads (Pelobates spp.) are highly specialized burrowing and nocturnal species with a narrow ecological niche. We tested if age-related parameters are responsible for sexual size dimorphism and size differences in two syntopic populations from the Danube Delta Biosphere Reserve (Romania). The two populations differed significantly in size and body mass, and showed distinct sexual dimorphism, with females significantly larger in P. fuscus, and males slightly larger in P. syriacus. We also found differences in age structure: P. fuscus was on average 5.0 yr old, whereas P. syriacus was on average 7.4 yr old. Both species reached sexual maturity at a similar age. Different growth rates before sexual maturity and differences in energy allocation between growth and reproduction after sexual maturity are responsible for the differences in adult size between these syntopic populations. Our results suggest that growth allocation between species and sexes cannot be explained solely on the basis of age-related parameters.