These muzzles must be fitted loosely.
There should be 2 fingers room at the front and 3-4 fingers at the back.
The chin should not touch the back of the muzzle.
Try putting a little hay or grass through the mouth hole to check the chin is not rubbing at the back.
The cheek pieces should be as close to vertical as possible. If they are pulling back at an angle, the muzzle is too tight.
The leather headpiece can have extra holes punched in if necessary.
If your pony / horse is not used to wearing a grazing muzzle, it is advisable to introduce it slowly while they learn to eat with it on.
If you have a Houdini that can get the head strap one its ears, try putting a plait in the mane and thread the head strap through it!