Here are a few things to consider: 1) Are you stitches squished up on the DPNs so they won't slip off? If so, then you're not seeing the actual size of the hat, and 2) the hat will stretch, of course; that's what the ribbing is for, but it may very well be that it's too small.
You might want to knit a swatch in the round (video here: http://www.knittinghelp.com/video/pl...r-gauge-swatch
) to check your gauge. HOWEVER, hats are usually such smallish projects that it's almost not worth knitting a gauge swatch because if you're unhappy with the size, you can usually start over quite easily. That's just me, though. You
may find it more efficient and less frustrating to just go ahead and knit a gauge swatch and then do your hat based on it.