Deviations from random mating in frogs are often explained by two different size-based patterns. The large-male mating advantage predicts that males found in amplexus with females will be larger on average than non-amplectant males, whereas size-assortative mating predicts that males and females found in amplexus will maintain an optimal size ratio. Both these pairing patterns are consistent with a female mating preference for larger males, or for males of a given size relative to the choosy female. I examined pairing patterns of two species of Neotropical hylids, Agalychnis callidryas and A. moreletii for three consecutive breeding seasons in Belize, Central America to evaluate whether mating behavior was influenced by either a large-male mating advantage or size-assortative mating. For each species, I compared size traits between amplectant and non-amplectant males, and within amplectant pairs, and I quantified fertilization success for each amplectant pair. For both species I found evidence of deviations from random mating by size, but the nature of the deviations varied between species and among years. The proportion of eggs fertilized was consistently high among years for both species and there was no relationship between fertilization success and the size ratio of amplectant pairs. These data are consistent with female mate preference, but a role for male–male competition cannot be excluded. My findings suggest that mating patterns may be density-dependent and that the nature and intensity of sexual selection may be increased by extreme environmental conditions.