A theoretical mechanism of evolutionary body size change is proposed, based on the allometric scaling of energy intake and metabolic expenditure rates with body mass over ontogeny. Genetic or environmental changes that affect these allometries are hypothesized to incidentally change the balance of selective forces on adult body size. A new descriptive term, alloenergetic, relating to such changes is proposed. This new mechanism is termed the size-energy differential (SED) hypothesis. The SED mechanism is shown to be a potential factor in both evolutionary size increase and decrease. Random alloenergetic mutation under the SED hypothesis favors size increase on the average. The SED hypothesis predicts that adult mass, among species, should show a positive association with mass-specific net-production rates and mass-specific food conversion efficiencies during ontogeny. The SED hypothesis also generates realistic, positively skewed distributions of species body mass on a logarithmic scale, and may suggest a new approach to the explanation of "Cope's Rule" of evolutionary size increase. Problematical aspects of the model and its domain are discussed. The importance of both size-specific net production and sizespecific mortality risk in determining the actual optimal adult size is emphasized.