Size limited brood capacity is common among species with hard exoskeletons or shells. In these species, brood size is limited by the physical capacity to hold offspring. Here we present evidence that brood size is limited by physical constraints in Sphaerium striatinum, a small brooding bivalve. Sphaerium striatinum is a sequential brooder and produces offspring throughout the year. Offspring are brooded in marsupial sacs located on the inner demibranch. In an unconstrained brooder one would predict that brood size would increase as a function of adult length cubed, a volumetric relationship. In S. striatinum, brood size increases as less than a function of adult length squared. We demonstrate that brood size is limited by two general constraints: marsupial sacs and the retention of extra-marsupial offspring. The number of marsupial sacs increases as less than a function of adult length squared. This relationship may be a result of physiological process such as feeding and respiration. Offspring size at independence is a crucial factor in determining offspring survivorship. The retention of extra-marsupial offspring promotes growth inside a safe environment and increases survivorship upon independence. However, the exponent relating brood size to adult length is significantly less for adults that contain extra-marsupial offspring than compared to adults that do not contain extra-marsupial offspring. Although the evolution of brooding in S. striatinum has resulted in severe constraints on brood size, the benefits of brooding outweigh the cost of limited brood capacity. We discuss our results in relation to brooding strategies and size limited brood capacity in other brooding bivalves.