Crop seed size affects the competitive interaction between spring wheat and wild oat.� However, the plant growth resources affected, and mechanisms associated with the process are not known.� The effect of seed size on spring wheat - wild oat competition was assessed using a mechanistic system involving yield and its determinants in these species.� Large and small seed size classes of �McNeal' spring wheat were evaluated under different seeding rates and wild oat densities during 1999-2001 near Kalispell, MT.� Interactive linear structural models based on ontogenic diagrams were constructed for each seed size system.� Spring wheat grain yield was primarily determined by spikes m^-2 and kernels spike^-1 when wheat was established from large seeds, but only by spikes m^-2 when derived from small seeds.� Concurrently, wild oat seed production was determined by panicles m^-2 when grown in competition with wheat established from large seed, but by both panicles m^-2 and seeds panicle^-1 under the alternative scenario.� Plants derived from large seed had a noticeable effect on wild oat via a reduction in seeds panicle^-1.� Wild oat competition reduced wheat spikes m^-2 under both seed size systems. However, plants established from large seed compensated for this effect via a compensatory mechanism involving kernels spike^-1.� Non-genetic variations in crop seed size affected the competition between these species by altering their mechanisms of yield compensation in which seed number was principally involved.