Under most circumstances, large body size confers a higher fitness and is positively selected, whereas selection against large size is empirically poorly documented. Physiologically, according to the 3⁄4 power law, larger animals have lower relative but higher absolute energy demands, such that large body size may become disadvantageous, particularly under fast locomotion in food-limited environments. After a period of initial feeding on different sugar concentrations, we investigated size-dependent energy content (reserves) at baseline and of females unflown (i.e. resting) or flown for 18 h in two (replicate) insect species: the yellow dung fly Scathophaga stercoraria and the yellow fever mosquito Aedes aegypti. Tethered adults of various sizes were tested in a flight mill. In both species, teneral glycogen, sugar, and lipid content increased with sugar availability, and isometrically or even hyper- allometrically (slope > 1) with body size. Activity treatment also revealed the expected consumption effects. Both species increased their flight distance with sugar supply, although only larger mosquitoes flew longer. Crucially, larger females of both species disproportionately exhausted more glycogen and sugars (but not lipid) during flight. The mosquitoes appeared to adjust their flight more finely to their size-dependent energy reserves at all sugar availabilities, whereas, in the dung flies, size-dependent energy demands were detectable only with a low but not with an overly high sugar supply. Although we found a greater absolute and relative locomotory energy demand for the larger flies, which is in agreement with interspecific patterns in insects, this was (more than) compensated by their greater baseline energy reserves, resulting in the greater net flying capacity of larger individuals. Consequently, we found no evidence for energetic mechanisms limiting the performance of large flying insects under food limitation. The differences between the two species presumably relate to mosquitoes inherently being long distance flyers and dung flies being short distance flyers.