As nutrition in the Western world has increased, so have body sizes, including feet sizes. According to podiatric historian William Rossi, "People are getting taller and heavier...and their feet are getting larger in proportion to their bodies."
According to Time, at the beginning of the 20th century the average woman wore a shoe sized 3.5 or 4 and a 5.5 in the '40s through the '60s. By the '70s, the average woman's foot was a 7.5. Today, the most common shoe size for American women is between an 8.5 and a 9.
Time quotes the U.K. tabloid Daily Mail, citing that “medical experts believe that eating high-density foods such as pizza and processed foods during puberty stimulates the growth hormone,” adding that an excessively heavy child can flatten and spread his feet by putting too much weight on them.