I read reviews that said these slippers run a bit small. I’m a size 10 and I don’t like snug slippers so I bought the XL (11/12). I’m so glad that I did. If I push my feet into the toe, I have a little space in the back. Worn comfortably, my feet feel cozy, with just a bit of wiggle room, if you want a plusher, snugger fit, don’t size up because they aren’t plush enough to really hug your feet at the larger size.I’ve noticed some photos of the back edge seam opening. I wonder if this is because See full review