Physical and chemical defenses of seeds are essential for plant survival and reproduction. By protecting their seeds from herbivores and other threats, plants can ensure that their offspring have a bet­ter chance of surviving and growing into adult plants. Plant seed-size variation can affect their ability to allocate defense resources and nutrients. Smaller seeds tend to contain fewer resources and are likely to be more constrained in resource allocation to defense or to nutrient supply for the germinating seedling than larger seeds. In this study, we hypothesized that (1) the trade-off between physical and chemical defenses and reserve materials (in this study TNC content) in seeds is related to the allocation of those resources to seeds, 2) the direction of allocation of these resources is mediated by seed size. Based on seeds of 77 woody plant species collected in the Kórnik Arboretum located in western Poland, we measured several physical and chemical seed traits: seed mass, total phenols, tannin content, crude fiber, lignin content, and total non-structural carbohydrates. We revealed that chemical defense (phenol content) and total non-structural carbohydrates increased with seed mass, whereas physical defense (fiber content) had the opposite effect. We found that the trade-off between physical and chemical defense in plants was often mediated by seed size.