From the foot wear news article:
Just like your waistline, the size of your foot can increase in width. Unfortunately, a regimen of diet and exercise will not likely get it back in shape. The causes can range from expansion due to heat, standing for long periods, or even pregnancy. Advancing age can also contribute to changes in foot size, both length and width.
Although less common, there are also those whose feet are narrow, an issue among children as well as adults and seniors.
Read more at the link: