If you work a yarnover between two knit stitches it's very simple, just wrap the yarn around the needle in the direction you wrap your yarn when you knit.
However when you are working a yarnover between and knit and purl stitch it becomes trickier. In order to wrap the yarn around the needle you have to bring the yarn to the front of the work (in order the work the next purl stitch) and then wrap it around the needle. This uses more yarn and therefore creates a larger yarnover.
It is possible to wrap the yarn in the other direction around the needle, this creates a stitch that is mounted backwards on the needle so you will have to knit into the other side of the yarnover on the next row. It does however create a smaller yarnover.
I've put a small video together showing these two different methods below: