Why are insects so energy-efficient while flying? Is it because of their light weight and aerodynamics or due to very efficient biochemical transformations (food->energy)?
Insect flight muscle is capable of achieving the highest metabolic rate of all animal tissues, and this tissue may be considered an exquisite example of biochemical adaptation.
Locusts, for example, may (almost instantaneously) increase their oxygen consumption up to 70-fold when starting to fly. In humans, excercise can increase O2 consumption a maximum of 20-fold, and for birds in flight the figure is about 10-fold (Wegener, 1996; Sacktor, 1976).
As Wegener (1996) has put it (in his definitive paper):
Flight is powered by ATP hydrolysis, and these impressive metabolic rates are achieved by very effective control of ATP hydrolysis and regeneration.
Wegener, G. (1996) Flying insects: model systems exercise physiology Experientia May 15;52(5):404-12. (See here)
Sacktor B. (1976) Biochemical adaptations for flight in the insect. Biochem Soc Symp. 1976;(41):111-31. (See here)
The smaller an animal is the easier it becomes for it to fly. That is because surface area increases to the second power of the diameter of the animal whereas mass increases to the third. So the larger a thing is the more mass per surface are it has.
And since insects tend to be small they tend to be good at flying.
As for any other reason, I don't think insects are any more energy efficient than say, birds.