Six diets prepared from a basal diet and either 30% corn starch or 20% sucrose or 8% rapeseed oil or 30% of a mixture of fibre-rich ingredients or 8% rapeseed oil and 30% of fibre-rich ingredients (diet 6) were fed to 45 (stage 1), 100 (stage 2) and 150 (stage 3) kg body weight pigs in order to quantify the effect of body size on digestibility of feeds. Digestibility of energy and nutrients and urinary and methane energy losses were measured for diets and calculated (by the difference method) for corn starch, sucrose, rapeseed oil and fibrous ingredients. Animals were fed close to ad libitum. Digestibility coefficients (DC) of nutrients and energy of diets were significantly higher at stage 2 than at stage 1. The improvement was small between stage 2 and stage 3. On average for the six diets, the DC of energy was 82.6, 85.0 and 85.8% at stages 1, 2 and 3, respectively. However, the variation in digestibility of diets with body weight was greater when dietary cell wall content was increased. The effect of body weight on energy value of diets estimated from DE measurements was attenuated by higher methane (0.5, 0.7 and 0.8% of DE intake at stages 1, 2 and 3, respectively) and urinary energy losses in heavier pigs. As a consequence of changes in DE values of diets with body weight, DE values of sucrose, rapeseed oil and fibrous ingredients were higher in heavier pigs. In 100 kg pigs, the DE content of corn starch, sucrose, rapeseed oil and fibrous ingredients averaged 17.7, 16.1, 37.7 and 8.7 MJ per kg dry matter, respectively. Results obtained on diet 6 indicate no evidence of digestive interaction between fat and fibre.