Intensive harvesting of the sea urchin Paracentrotus lividus, a crucial grazer in Mediterranean rocky reefs, selectively removes commercial-sized individuals and produces size-truncated populations dominated mainly by medium-size individuals. Whether this demographic reorganization alters grazing pressure on macroalgal communities remains poorly understood. Grazing at the population scale emerges jointly from per-capita consumption, size structure and density of the population. We quantified individual algal consumption of medium (i.e. under commercial, 35-45 mm test diameter) and large (i.e. commercial, >50 mm) sizes of sea urchins under standardized laboratory conditions, using individuals from two contrasting Sardinian populations outside marine protected areas: a truncated population (exploited condition) and a well-conserved population (unexploited condition). Trials were run in the cold and warm seasons to capture different physiological states of the gonadal cycle. We analysed size-normalized per-capita consumption, through a fully crossed three-way ANOVA (population x size x sampling period). Also, grazing at the population-scale was estimated by scaling absolute consumption to the respective population demographic structure under i) the same density (controlled conditions) and ii) the densities observed in the field. Results indicate that population-level grazing pressure cannot be inferred from per-capita consumption alone, but rather emerges from the combined effects of individual consumption rates, population size structure, and density. This highlights that fishing-induced size truncation may reshape macroalgal grazing through demographic reorganization, with effects that can differ or even reverse between controlled and field-density scenarios.