If it's flat strap goods you can oil it and leave it under tension for a few days and it'll stretch somewhat. Getting it wet should work too. If it's sewn more than just at the attachment points forget it, stitching won't stretch.
Is there any way to exchange it for the next size up?
when punching holes in the ends, consider using a drill bit in the correct size - makes much cleaner holes and is far easier than using a hole punch IMO. I have two hole punches and i never use them, using a drill is infinitely easier.
It should stretch anyway, mine always do. Then they are too big lol. Either make the crown longer until it stretches on its own, for try soaking it in plain water till its soft and pull the heck out of it with your hands then put it on the horse.
Spray it with rubbing alcohol until it is good and wet, then put it under tension until dry and see if that worked.
Maybe hang it up and hang something heavy from the other end.
If it's already heavily oiled, this won't work.
Your beliefs don't make you a better person, your behaviour does.