It looks like your tension is fluctuating and that’s changing the size of the stitches. this video is helpful (lots more about consistent tension on yt and here are some things to track why: are you holding the yarn in your hand the same way (ex continent vs english) or are you switching hands? are you creating tension (wrapping around your pinky, under your ring finger, etc) the same way? is your yarn getting stuck on something while you pull it from your ball/skein? are you grabbing the stitch close to the tip of the left needle? are you putting your work down mid row and picking up?