Efficiency is power out compared to
power in, so you need an aerofoil design that has a good lift/drag ratio. A
high aspect blade (long and narrow) will give you low induced drag and so it
needs low engine power to overcome drag. As propellers operate at big angles of
attack to produce thrust, a low induced drag will give you a good efficiency.
If you look at your light aircraft
that is exactly what they have. Fairly long narrow blades (high aspect ratio) and
good efficiency, so you don’t need a big engine to drive them.
Unfortunately they don't move a lot
of air backwards, so although they are fine if the aircraft weight is low, you
need more disc solidity, if you need more thrust. The minute you increase disc
solidity (with wider blades) you are reducing aspect ratio and losing
efficiency. You will need a stronger engine to overcome the higher drag.
Power absorption is the same as disc
solidity in that it reduces efficiency.
If you have a powerful engine you are
wasting it if you put a high aspect ratio (low disc solidity) propeller on it,
because you could drive it at idle. You might as well use the power available
by increasing the disc solidity and produce more thrust.
Increasing disc solidity or absorbing
more available power will always be contrary to efficiency. The designer has to
decide what he wants, you cannot have both.