In an asexually reproducing plant (Potamogeton pectinatus L.) the competitive advantage of a larger propagule over a smaller one was determined by a single parameter (, the competitive asymmetry coefficient), independent of sowing density or average propagule size. Competitive advantage was determined by the relative size difference between propagules instead of their absolute difference. Productivity per germination site increased with the number and size of propagules present, even at densities high enough to result in decreased per capita yield due to plant competition. Both the number of propagules produced by a plant and their size increased with increasing per capita resource capture. Therefore, propagule size was neither equal to the size of the propagule from which the plant germinated nor independent of resource status. Our results clarify what assumptions should be made in ecological and evolutionary models dealing with competition for resources between seedlings. In particular, the use of relative size differences instead of absolute ones should lead to the evolution of smaller propagules than those expected if competitive advantage grew with absolute propagule size.