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.