The study presented in this paper focuses on the fundamentals behind size segregation in particle beds in mineral processing contexts. It investigates the proposition that size effects can be modelled on the same basis as that used to model density stratification, namely that stratification is driven by the reduction in the potential energy of a bed that is brought about by stratification. This conception requires that segregation of a mixture of differently sized particles brings about a change in the packing density in the bed. To test this conception and to investigate the relationship between packing density, size segregation, and the bed's potential energy, seven particle systems were studied in which differences in particle size were the primary variables; all particles had essentially the same density and shape. Variations in packing density and particle concentrations within beds stratified in a laboratory jig were determined from 3-D images of the beds generated by X-ray computed tomography. Three primary conclusions were drawn from the study findings. (1) Packing models derived from the literature provide an adequate means of describing the relationship between the local size mix and local packing density in segregated beds. (2) Size stratification is not driven by a reduction in the potential energy of the particle bed in jigs. (3) In many practical contexts, the variation in packing density within a stratified particle bed may be small enough to neglect.