I have 2different swaddleMes and they said they were the same size, but 1runs smaller I've noticed. (It's smaller in the wrapping of the torso.) My nephew was only 8 lbs. and his swaddleMe was small too. Have you tried not making the Velcro meet exactly on the pads, offsetting it could help loosen it up.
I've noticed this exact thing. We have five of them--2 are a bit small and 3 (came together in a pack) are a bit large, even though they're all marked as the same size.