Take a look at the weight of the yarn called for in the pattern. It might be that your yarn is a bit smaller in gauge than the yarn called for. Also, I have learned that hats stretch more than you think they will. If you have cast on the right # and the right yarn, I would personally knit for an inch and see if the bottom doesn't seem to be bigger once you get past the needles.
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Oddball Pet Snuggles
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