Just as people are all different, so are knitters and crocheters.  Some people like to knit or crochet very tightly (high tension on their yarn or thread), while others like a more loose and open tension.  For this, one should always determine their own gauge (stitches per inch) so they can purchase the correct yarn.  Also keep in mind that when you use yarns or threads other than what is called for in a pattern, the sizing will not be accurate unless you know your gauge.

A project calling for baby yarn has much smaller gauge than one calling for baby sport yarn.  If you follow a pattern that calls for baby sport yarn, and you use baby yarn (whatever your gauge), the finished project will be much smaller than the pattern estimates. 

To find your gauge, you must crochet or knit a sample piece of the pattern using the yarn you would like, making it approximately 2" long.  Count the number of stitches it takes you to achieve one inch.  That will be your gauge.  So if a pattern using worsted weight yarn suggests your gauge should be 7 stitches per inch and your gauge is actually 10 or 12 inches per stitch, you need to either change your needle or hook size by one size up, knit or crochet with less tension, or use a thicker yarn. 

With time and practice, you can adapt any pattern to get the size you want, with the yarns you want.