We measured resting metabolic rate (RMR), daily energy expenditure (DEE) and metabolisable energy intake (MEI) in two breeds of dog during peak lactation to test whether litter size differences were a likely consequence of allometric variation in energetics. RMR of Labrador retrievers (30 kg, n=12) and miniature Schnauzers (6 kg, n=4) averaged 3437 and 1062 kJ/day, respectively. DEE of Labradors (n=6) and Schnauzers (n=4) averaged 9808 and 2619 kJ/day, respectively. MEI of Labradors (n=12) was 22448 kJ/day and of Schnauzers (n=7) was 5382 kJ/day. DEE of Labrador pups (2.13 kg, n=19) was 974 kJ/day and Schnauzers (0.89 kg, n=7) were 490 kJ/day. Although Labradors had higher MEIs than Schnauzers during peak lactation, there was no difference in mass-specific energy expenditure between the two breeds. Hence, it is unlikely that litter size variation is a likely consequence of differences in maternal energy expenditure. Individual offspring were relatively more costly for mothers of the smaller breed to produce. Therefore, litter size variations were consistent with the expectation that smaller offspring should be more costly for mothers, but not that smaller mothers should per se invest more resources in reproduction.