Hey, that's my pattern :waving:
If you go by the gauge on the yarn ball, then yes it would appear that they would turn out big -- but that's a stockinette gauge measurement, whereas the entire hat is knit in a cable pattern, so the gauge would be different. Cables pull together a lot, so I think you'll find that it turns out much smaller than 18". I would measure a finished hat for you but I've given them all away!
If anything, before trying to alter the pattern, you might want to knit a gauge swatch in the cable pattern to be safe.
If anything, people have reported that when they finish, the hat looks too small rather than too big -- but the cable pattern makes it very stretchy. In any case, if you do knit it I would love to hear your feedback.