In a dairy herd, all cows do not use a similar ration (amount, quality) in the same way. At the same level of performance, body reserves and live weight, they consume different amounts of the same ration, meaning that they do not have the same feed efficiency (FE). The differences in feed efficiency can be up to 10% of the feed intake difference between the most and least efficient cows. The main challenge is to be able to estimate FE without measuring individual feed intake, which is only available in a few research facilities. Another way consists in understanding the drivers of FE and directly use these drivers to estimate or improve it. This work focusses on digestion differences, which is known to be a driver of FE in dairy cattle. We hypothesised that the digestion differences can be approached indirectly thanks to gas analysis using Green Feed (GF) technology and the change in rumen volume, using 3D imaging technology. The 3D image data of the animals were used to calculate the volume of the abdomen (AV), considered as a proxy for rumen volume. For this purpose, data from 60 lactating Holstein cows was used, receiving successively two diets based on a low (L), or high (H) level of Neutral Detergent Fibres (35 vs 42%). L and H rations contained 14.5 and 16.7 crude protein, 0.83 and 0.84 UFL and 93.0 and 101.9 g/kg of PDIE, according to INRAE feeding system. The AV increases linearly with dry matter intake (DMI), which confirms the interest of AV. The comparison between the two periods indicated that the rumen volume increased with the diet H, which is consistent with the notion of rumen fill due to fibre content of diet. At the same level of DMI (L diet), the AV increases with FE of the cows, with a significant negative coefficient of Residual Feed Intake (RFI; -21.8 l/kg) to explain AV with DMI and RFI. If CH4 yield (g/kg dry matter intake) was positively connected with feed efficiency, no significant relation was found between CH4 yield and AV/DMI. These results suggest that FE seems driven rather by digestion and less by metabolic drivers. The better digestion for the most efficient cows could be due to a longer residence time of the ration in the rumen.