1 The trade-off between seed mass and the number of seeds a plant can make for a given amount of energy underpins our understanding of seed ecology. However, there is little information on the magnitude of the fecundity advantage of small-seeded species over an entire plant lifetime. 2 We compiled data from the literature to quantify the relationships between: (i) seed mass and plant size (because the photosynthetic area of a plant determines how much energy is available for allocation to seed production); (ii) seed mass and plant reproductive lifetime (the number of years a plant has to produce seeds); and (iii) seed mass and the number of seeds produced per individual per year, and per lifetime. 3 Seed mass was positively related to all measures of plant size (canopy area, plant height, stem diameter, plant mass and canopy volume). There were also positive correlations between seed mass and time to first reproduction, plant life span, and reproductive life span. Thus, although small-seeded species produce more seeds per unit canopy area per year than large-seeded species, large-seeded species tend to have larger canopies and more reproductive years. 4 These patterns accord well with independently gathered data on annual and lifetime seed production. The negative relationship between seed mass and the number of seeds produced per year was much shallower on a per individual basis than on a per unit canopy basis. Seed mass was not significantly related to the total number of seeds produced by an individual plant throughout its lifetime. 5 Our previous understanding of seed mass as a spectrum from production of many small seeds, each with low establishment probability, to a few large seeds each with higher establishment probability, was missing some important elements. To understand the forces shaping the evolution of seed mass, we will need to consider plant size and longevity, as well as seedling survival rates and the number of seeds that can be produced for a given amount of energy.