Abstract Teneral reserve components (soluble sugars, glycogen, lipids), as well as proteins, of laboratory-reared yellow dung flies (Scathophaga stercoraria) are shown to increase hypo-allometrically (slope < 1) with body size. This is also true for sugars and glycogen of field-caught reproductive flies, the lipid content of which however, increases hyper-allometrically (slope > 1), probably related to foraging or reproductive activity. The longevity, or starvation resistance, of adults subsisting exclusively on their teneral reserves equally increases with body size, and this positive relationship remains when flies have access to sugar, pollen or prey for 24 h after emergence. Energy use, by contrast, is generally not body size dependent, except for lipid (and possibly glycogen) utilization when flies were completely starved. All the results obtained are independent of sex after controlling for sexual size dimorphism. These net results clearly support the greater energy efficiency of larger flies (Kleiber’s law), whereas only weak evidence is obtained in support of the contrasting hypothesis of greater absolute energy demands of larger individuals, possibly because such effects are likely masked by stronger size-dependent anabolic effects.