U.S. shoe makers including Stuart Weitzman and Cole Haan report average sizes are creeping up. And retailers are watching the extended-size market carefully. Nordstrom has seen strong sales of larger sizes, says Anne Egan, national merchandise manager for salon shoes. It has held special in-store events for extended-size customers, including women who wear up to a size 14 and men who wear up to a size 20. Long Tall Sally, a U.K.-based apparel and footwear retailer that gets almost half its sales from North America, sells the most shoes in U.S. sizes 12 and 13, says Chief Executive Andrew Shapin. Size 15, added earlier this year, now makes up 10% of its footwear business.

No matter how big or small your feet, though, your shoes could be hurting them—or even causing permanent harm. In the U.K study, involving 2,000 adults, more than a third of men and nearly half of women admitted buying shoes that didn’t fit properly. Shoes with a narrow “toe box,” the industry term for the front part of the shoe, can push the big toe in and create or accelerate a bunion, says Steven L. Haddad, a Glenview, Ill., orthopedic surgeon and president of the American Orthopedic Foot and Ankle Society. It can also constrict the toes, resulting in what are known as “hammertoe deformities.”