Oh my criminy! What a dork! Yes, it is acrylic. I don't know why I had it in my head that it was cotton, must be one of my other projects. Yarn on the brain. Hee, hee, how embarassing
I don't know if the yarn difference was the reason it was bigger than I expected, if I remember correctly (which I obviously cannot guarantee) my gauge was correct, but I may have made my stitches looser as I went along, I have done that on other projects too. The picture on the site looks a lot tighter than mine.