The perils of gaugelessness

I've just started the decreases for the Medusa hat, which means I'm nearing the top. I tried it on my own head for fun this afternoon...and it fit pretty well. Ack. Looks like this hat is destined for an adult, rather than for Sylvia. I sort of did a gauge swatch for this--just in stockinette, not in pattern (those little snakes, though simple, are a little time consuming to make)--and things were looking good. I'm not sure how I went so astray...