Small mouth - small flies!
 

The headline contains some truth in the sense: When trout/graylings eat tiny surface-food, they only nead to open their mouth to a narrow slot and sip the fly in. If one presents them with a fly with a broad hackle - then they canít suck it in through their narrow mouth.
Vince Marinaro in his book "A modern Dry-Fly Code", 1950 showed how he solved the problem.
For his famous 'Jassid' he uses:

 
Hook Model Perfect, regular size 16.
Hackles Two short-fibred black cock-hackles wound as a palmer up over the shank, and then cut on top and bottom, so that the fibres only point out to the sides.
Wing An eye-feather from a Jungle Cock tied down, so it covers the shank.

© Preben Torp Jacobsen. 'Flyleaves', 1996.

It floats perfect in the surface, and by this imitates the small, black insects caught in the surface.