enjaybel3 is correct. You can increase the efficiency of the engine, make more power, and in fact reduce amount of fuel required. This is usually measured by the BSFC on any generic engine dyno.
Having said that, I do agree that you can only get so much power out a maximum amount of usable fuel. As teams like Mercedes strives to increase the thermal efficiency of their engine, I have to believe their improvements are measured in very small percentages. Since 2016 they have gained like 150 horsepower? Of course there is also the electric power. Theoretically they could see significant gains in "power" by how they deploy their electric energy, and it should cost them no more, or less fuel than by just using the ICE.
To me, for Mercedes to have such a massive advantage all these years (except when Ferrari "cheated"), they would have to be either using more fuel than allowed, more peak electric power than allowed, or more energy per lap than is allowed.
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