Make sure your ski goggles are in right size first. If it is, you can try to loosen the strap to see whether it can fit you better. If it doesn't work, you can take your goggles back to the optician and ask him or her to adjust it for you.
If the strap can be loosened, try that. If there is nothing you can do for it, take it back to the place where you bought them and ask the staff there to adjust it. Otherwise you need to get a new pair.