Hat came out too small

I just made this hat: http://www.ravelry.com/patterns/library/hat-31-cable-gingerbread

It came out lovely, but much too small for an adult. It’d be good on a young child. This is the first time I’ve knit a hat and I haven’t knit many things that require very accurate sizing. I’ve never done a gauge swatch and this pattern didn’t actually have one I could do if I had wanted to. If I ever hope to knit for other people I need to get this sizing issue taken care of. If the hat came out too small does this simply mean I knit too tightly? If so, would you suggest I learn to knit more loosely or should I just change the size of my knitting needles? Will doing a gauge swatch insure my items come out the correct size?

There could be several reasons. It could be you are a tight knitter or used a different weight or needle size than suggested.

Did you use the same yarn/weight and needle size? What yarn did you use? Are you normally a tight knitter?

Yes, I used worsted weight yarn (Vanna’s Choice) and size 6 needles as the pattern instructed. Honestly I don’t know if I’m a tight knitter, I’ve done a couple of dog sweaters but those had you alter the pattern a bit to size (knit for however many inches you need to for your dog) and I’ve done one baby sweater purely for educational purposes (has never gone on a baby and I don’t have enough experience with babies to know if it would fit a 6 month old as the pattern says it should). I’ve done a few pairs of socks, again you measure for length, but they have all fit around just fine.

Well, it’s likely you knit tight then since you used the same yarn. Here’s a standard size chart that might help.
http://www.craftyarncouncil.com/babysize.html

If you do feel it’s too small you could go up a needle size which would probably be the easiest way to upsize. Otherwise you have to keep the pattern in mind and where to add increases. Cables naturally pull in like ribbing so that’s not uncommon.

Measure the gauge of items you’ve knit and remember the size needles and yarn you’ve used. Start paying attention to the gauge on the band to see if you’re tighter or looser. I prefer to adjust my gauge by needle size, I figure I’m most consistent doing it the way that feels best.

That pattern really wasn’t written to fit. No finished size given, no gauge given. That makes it hard to adjust beforehand.

If it takes a lot of force to move the needle through your stitches, you probably are a tight knitter. There’s nothing wrong with it, but you should go up at least one needle size. My sister mentioned once that she knits so tight that the pattern she had called for a pair of 7s and she had to use a pair of 10 1/2 to achieve the same gauge.