The plantar fascia band on the bottom of the foot is similar to the bow-string of an archery bow. Over many years the plantar fascia band may gain length. Partially this is true because of bony growth called a heel or bone spur.
Another result of this fascia lengthening is that people think their feet are growing longer, but they didn't "grow". With the loosening plantar fascia band the foot elongates or spreads out. This is especially common if a person has put on a lot of weight.
Oddly enough, the most flattening of the foot occurs on hard flat man surfaces. If you step in sand or soft earth, even a flexible foot is firmer than the ground, so instead of the foot giving to the ground, the ground gives-in to the foot. This is why Footform custom orthotics mimic the natural footprint in sand, so that the foot remains in a natural state while weight-bearing.