As you knit, you stretch (and then relax) each stitch.
the stretch gets passes along to the next stitch in the row (think of the WAVE at a stadium)
By the end of the row, there is no where for all the stretch to go!
(and you end up with an over sized stitch.
start the row with extra snug stitches.
pull stitch 1 and stitch 2 tight.
then as you turn, and come back to then of the row, (the last 2 stitches), these stitches have less ease, and can absorb some!
Snug up stitch 1 and stitch 2 and in 2 rows, your selvages edges will be looking better!
the hard part is to get into the habit. (and its harder to do if you already knit tight!) tighten up stitch one and two, let the others be easy!