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Three chicken breeds, including one commercial broiler (CB) and others from the Norwegian gene bank (Barred Plymouth Rock and Jaerhøn) differing in body size were raised under intensive (ad libitum feeding) and extensive (restricted feeding) systems from 7 to 20 weeks. Body weight (every fortnight), carcass, breast, thigh and abdominal fat weights were recorded. CB chickens had heavier live weights, grew more rapidly and had heavier carcass components at slaughter under intensive conditions, but lost more weight under extensive conditions than the other two breeds. Intensive birds showed a higher degree of maturity than extensively reared birds. Males expressed their growth capacity much less than females under extensive conditions. Size scaling narrowed the gap between the performance of CB chickens and other genotypes but the specific genetic effect still remained. Therefore, both genetic and underlying size differences should be considered in breed comparisons, particularly when chickens are subjected to different feeding regimens.