There are two possible reasons why your finished size doesn't match the pattern: 1. The weight of the yarn and the size of the crochet hook you use might be different than what the pattern says.  Solution: Use the yarn and hook that the designer says to use. If you can't find the exact yarn, you can ask the designer or the yarn shop to suggest a similar one. 2. Crochet tension is either too tight or too loose. Solution: Use a smaller or larger hook to get the correct tension.