When examining the mass-related energy expenditure of children of differing body sizes, most empirical research studies have been based upon the application of identical absolute workloads (Rowland, 2012). Consequently, the accepted interpretation of such data may be an “artefact of the laboratory exercise testing protocol” (Rowlands, 2012, p. 502). It is therefore pertinent to compare the VO₂ responses of children of differing body size at a relative exercise intensity. This study compared the effect of body mass (normal weight [NW] cf. obese [OB]) on exercise economy during relativised moderate- (90% gas exchange threshold [GET]) and heavy (40% delta [∆]) intensity treadmill exercise, in children aged 8 to 10 years. Eighteen healthy NW- (body mass index [BMI]: 18.5 ± 1.0 kg•m²) and 19 OB children (BMI: 25.2 ± 2.5 kg•m²) volunteered for this study. Participants took part in two laboratory-based exercise sessions, at least 72 hours apart: i) a maximal graded exercise test to peak oxygen consumption and ii) a submaximal exercise test, wherein participants exercised for 6 minutes at 90% GET and 40% ∆ each, separated by a 5 minute recovery period. When overall body mass (mL•kg^-1•km^-1) and stride frequency (mL•kg^-1•stride) were considered, OB children proved more economical than NW children by consuming less oxygen (~13-15%) at statistically similar (P > 0.05) treadmill speeds (moderate and heavy bouts) and stride frequencies. Our highly novel findings contradict the currently accepted viewpoint that OB individuals elicit an inferior exercise economy compared to their NW counterparts (Browning et al., 2006; McMurray & Ondrak, 2011; Volpe Ayub & Bar-Or, 2003). The physiological underpinnings of the exercise protocol employed in this study have led to a more valid interpretation of exercise economy in children of differing body mass.