Fuel efficiency has another important parameter to consider.
Look for this two designs:
Second ship (the one from the right side) will get more acceleration for the same fuel used. Because it needs to push forward less weight.
And if we simply reduce amount of fuel in first ship (the one from the left) to make weights even for both - then first ship will have even less acceleration, since it lost fuel for nothing.
Plus you also need to carry out the entire small ship to its destination by some other bigger ship, and since the ship is lighter on mass due to smaller engine - the process of carrying out itself will use less fuel too. Starting from the very take-off from Earth already.
Often even big ships will actually use less fuel with Valiant engine, than they would use with Frontier engine instead. Though by "pure numbers" Frontier is more fuel efficient. But this "extra fuel efficiency" will be all wasted, when you need to simply burn more fuel to carry out the much bigger mass of the bigger engine itself.
How much big (heavy) the starting ship needs to be to benefit from bigger (but more fuel efficient) engine, instead of wasting more fuel to carry out that bigger engine - that depends not only on the starting ship size (mass), but on the ending ship size (mass).
For example if we have ship, which is 200 tons at the start and it will be 100 tons at the end (since other part of the ship is payload), then Frontier engine will be better. And if the same ship will be only 20 tons at the end - then Valiant will be better, since Valiant will get more acceleration for the same "180" tons (minus empty tanks weight) of fuel used.
So, Valiant "inefficiently" burned the whole "180" tons and still was more efficient at the end (giving more acceleration for the same fuel), than more efficient Frontier instead.
The difference in weights between Kolibri and Valiant is smaller, so the ship size when Valiant becomes better than Kolibri is also smaller. But it is not that small as one might think only by looking at how much fuel the engine needs to burn to get the same sum of thrust as another engine - not even close to that.