ABSTRACTThe size structure of plant populations has been characterized by two descriptors, size‐density relations and size inequality. In even aged natural or experimental populations, the logarithm of mean size is a negative function of population density, with a slope of –3/2 and a maximum intercept of 4.3. Size inequality, measured by the Gini coefficient (G.C.), ranges from maximum size homogeneity (G.C. = 0) to maximum inequality (G.C. = 1). These two descriptors have been applied mainly to land plants, and only once have they been empirically related, suggesting that the onset of self‐thinning (increase in mean size by decreasing density) reduces size inequality. We applied these descriptors and tested their empirical relations within populations of Iridaea laminarioides Bory, a red alga that, as other members of the Gigartinales and Gracilariales, exhibits coalescence of germinating spores, forming a clump of fronds with a single basal disc. Results indicate 1) lack of fit to the self‐thinning rule and transgression of the limit slope, 2) high size inequality values, 3) absence of a significant negative correlation between mean size and Gini values, 4) spatial ordering in the distribution of size inequality, with fronds decreasing in size towards the periphery of the coalescing discs, and 5) reproduction occurring mostly in the large sized fronds. In I. laminarioides transgression of the self‐thinning slope and high size inequality seems to result from spore coalescence and a growth pattern that allows the coalescing discs to grow into physiologically integrated fronds, the larger (and central) of which become reproductive. Therefore, rather than being considered exceptional, we suggest that transgression to the limit slope and high size inequality should be common among seaweeds with coalescing spores and physiological integration of individuals.