I also returned mine for this reason. I have muscular calves and the boots were just uncomfortable on me. I thought they were adorable, but, unfortunately, they didn't work for me. I don't know that the material they are made of is really stretchable. You could ask the store where you bought them if there is anything they can do. I know NM has stretched out a number of shoes for me in the past, but those have all been leather or suede. Good luck!
Jennifer