[mannykannot]

This is mentioned in the introduction here [1]. The version I'm looking at is an image, so I cannot easily copy-paste the relevant passage, but it seems to say that the efficiency gain comes from detonation causing the combustion to occur at a higher pressure than in the case of deflagration. Generally speaking, higher peak pressures increase the efficiency of heat engines, as this allows for greater expansion of the working fluid, and thus more work being done for the same fuel consumption.

With regard to your comparison, I guess this means that the detonation engine can have a higher pressure in the combustion chamber, together with a larger bell, a faster-moving exhaust, or some combination.

[1] https://arc.aiaa.org/doi/10.2514/6.2013-3971