There are no increases in that pattern, so it shouldn't change size.
I suspect that you're not consistently moving the yarn between the needles to the front to purl and back again to knit. :thinking: You'll give yourself yarn over increases if you work the next stitch with the yarn in the wrong place.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry