I think you mean 'displacement' (aka engine size).
A diesel engine with the same displacement has a smaller combustion chamber that a gasoline engine, due to its higher compression ratio.
Given that, a diesel has a more efficient thermodynamic cycle because the mixture of atomized diesel fuel and air can be compressed significantly more (before combustion begins) than a gasoline and air mixture.
Diesel engines take advantage of this by having higher compression ratios, ~18 to 22:1. Gasoline engines typically have compression ratios of ~8 to 10:1
Also, diesel fuel has slightly more chemical energy per volume than gasoline.