This is normal. For the majority of knitters purl stitches come out looser than knit stitches. You don't need to "fix" it, IMO, unless your purl stitches are so loose that you have actual holes in your work.
Your knits and purls will become closer in tension with time and practice. And don't forget that once you wash/block things all of the stitches will even out a bit and the difference in tension between the knits and purls will hardly be/not be noticeable.