The benefits of cooperative breeding include anti-predator defense, access to resources, and inclusive fitness via kin-selection. Smooth-billed anis (Crotophaga ani) are communally breeding birds in which co-breeding females lay eggs in a shared nest. Within-group relatedness is low, so kin selection is not driving this system. Group size ranges widely, and larger groups often exhibit intense ovicide, suggesting it is costly to breed in large groups. Assuming there are tradeoffs between group size and reproductive success, we predicted that with increasing group size i) group reproductive success would increase, while ii) per-capita reproductive success would decrease, and iii) the probability of offspring surviving to a subsequent developmental stage would increase due to the presence of additional caregivers. Using data from 364 nests, we show that larger groups laid, incubated, and hatched more eggs and fledged more chicks, whereas per capita reproductive success decreased with increasing group size across all four measures. Group size did not affect the probability of offspring surviving to a subsequent developmental stage, and anis frequently breed in larger-than-optimal groups. We conclude that communal breeding in large groups is likely driven by ecological constraints, factors that increase long-term fitness of individuals, and conflicting selection pressures among individuals trying to join groups and those already in groups. Our findings highlight the complex selection pressures that likely influence communal breeding in non-kin groups with highly variable group structure.