Terminology
Hello Marlboro,
When you speak about "higher rated engines using more fuel than lower rated engines" do you mean, figuratively speaking, a difference between an 850 cc three cilinder and a 6.5 litre V-12, or do you mean the 6.5 litre V-12 in basic form with 150 HP and the pepped up race version of 800 HP?
In aviation terms, do you mean one engine, in normal rated and derated version, or one big engine versus one small engine?
Putting a very big engine on a not so big airframe, could cause unnecessary drag, hence, higher than necessary fuel use.
Having identical engines on identical airframes, only one set derated and the other set not, there should be no difference in cruise burn.