On the premise that each genotype has its own inbuilt genetic size factor operating throughout growth, two formal genetic size-scaling rules have been proposed. A major outcome of the scaling rule for time variables is the concept of metabolic age. The corresponding concept for cumulated growth variables is degree of maturity. Genetic comparisons made at the same metabolic age or at the same degree of maturity are independent of adult size. By averaging over genotypes at each metabolic age or degree at maturity, mean size-scaled growth curves are obtained which provide a unified description of mammalian growth. Mean mammalian growth curves are presented for body weight, growth rate, body composition, heat production, maintenance requirements, food intake, and food efficiency. For any trait, deviations from these mean values or curves can then be studied; variation independent of genetic variation in adult size can be evaluated, and deviant species or strains identified. Unscaled expected values can also be calculated for any genotype of given adult weight whenever mean size-scaled values are available. Results based on genetic size-scaling are reviewed for genetic differences between species, between breeds or strains within a species, between sexes, and between sires within a breed.