Several experiments have shown that seedlings from larger-seeded species are better able to survive various hazards during establishment. Previous work has suggested a general mechanism might underpin this outcome. Larger-seeded species might tend to mobilize their metabolic resources over a longer period into the autotrophically functioning structures of the seedling. Consequently relatively more resources would remain uncommitted at any given time during the early period of the seedling's growth, and available to support respiration during carbon deficit. An important aspect of this larger-seed-later-commitment mechanism would be that at a given time, larger-seeded species would hold more resources uncommitted not just absolutely, but relative to the functional seedling structures that needed to be supported. Here we quantify, across a wide range of phanerocotylar species, an allometric pattern that supports the generality of a larger-seed-later-commitment mechanism as an explanation for superior performance by larger-seeded species in face of the hazards of seedling establishment. Larger-seeded species allocate relatively less to cotyledon area, reflecting the initial functional size of the seedling, and relatively more to dry mass per unit area of cotyledon, reflecting stored metabolic reserves. The shift in relative allocation is progressive, rather than seedlings falling into discrete morphological types. The allometry is similar whether considered as correlated evolutionary divergences (phylogenetically independent contrasts) or as correlation across present-day species.