Spirit got a marathon one recently, the arab size fits him fine and he's a pony sized arab. Would just say that though it's been brilliant for him when we tried it on Salladin it rubbed behind his ears, but he is a particularly sensitive soul and very prone to rubs.
My sister's Arab gelding takes a pony size headcollar, and he is fine in an Arab size Zilco - she cut the spare bits of strap off once it was adjusted to the right size. (Not bitless though).
Zahkira (GR Amaretto x Taffetta) Linda East Sussex