I'm told that very few people have both feet the same size! My left foot has always been longer and narrower while my right foot has been shorter and wider. I had both feet measured back when I was in my early 20's before they got deformed by minor things like 5 pregnancies, periodic bouts of overweight and genetics. At this point my right foot is still wider because the bunion there is more advanced.

I have met one person who has to wear an extra sock on the smaller foot!

You might want to have both feet measured by an expert shoe fitter (if you can find one) and buy footwear based on your larger foot. Of course since shoe lasts differ among models (and even more among manufacturers, and many change their lasts yearly), there's no substitute for a good try-on. It's a good idea to walk around for a while with your pack on.