Offspring sizes vary within populations but the reasons are unclear. Game-theoretic models predict that selection will maintain offspring-size variation when large offspring are superior competitors (i.e. competition is asymmetric), but small offspring are superior colonisers. Empirical tests are equivocal, however, and typically rely on interspecific comparisons, while explicit intraspecific tests are rare. In a field study, we test whether offspring size affects competitive asymmetries using the sessile marine invertebrate, Bugula neritina. Surprisingly, we show that offspring size determines whether interactions are competitive or facilitative – large neighbours strongly facilitated small offspring, but also strongly competed with large offspring. These findings contradict the assumptions of classic theory – that is, large offspring were not superior competitors. Instead, smaller offspring actually benefit from interactions with large offspring – suggesting that asymmetric facilitation, rather than asymmetric competition, operates in our system. We argue that facilitation of small offspring may be more widespread than currently appreciated, and may maintain variation in offspring size via negative frequency-dependent selection. Offspring size theory has classically viewed offspring interactions through the lens of competition alone, yet our results and those of others, suggest that theory should accommodate positive interactions in explorations of offspring-size variation.