Your yarn and hook size makes a big difference. Did you substitute something different for what was called for in the pattern? If you would use a fingering or baby sport in place of worsted weight called for in a pattern, for instance, your garment (or anything else) will be smaller than required. You can substitute yarns, but it has to be in the same weight class. Ask us here if you don't know how to do this. One of my biggest peeves with knitting/crochet magazines is that they use exotic yarn that's expensive and can only be gotten online from one company. It's certainly not going to be available off the shelf locally. I can and do substitute, but again, in the same weight category. The type of yarn, whether it's a natural fiber like wool or an acrylic, is also a big factor in size and how a yarn knits up.
Wools and acrylics are kind of like sisters. They're both girls (yarns) and in the same family (weight class), but they look and act nothing alike. One is a natural born child (natural wool fiber from sheep) and the other is adopted (manufactured acrylic, which is actually plastic).