For assorted reasons, almost everyone knows that their body has one foot bigger than the other. The difference is relative but for some it can be as much as half a shoe size. I knew someone who actually bought two pairs of shoes, with a half size difference, each time he went shopping and would swap them out. In the age of the internet there are actually websites where you can purchase different sized shoes if that is your wish.
The difference in foot size can be due to a variety of factors—handedness, injuries, and of course, the psoas. For one, and this is the most commonly stated reason is we are handed people, more dominant on one side of the body than the other. Also, and in the same context, we are not symmetrical beings. While the bones on either side of the body tend to be exact replicas of the other side, organically we are very different on each side (the heart is on one side, the liver on the other etc.). And the truth is every paired body part is going to be slightly larger or smaller.
But if our dominant side was the reason I should have a larger left foot as I am a lefty but it is my right foot that is larger. Some people opt to buy shoes for the larger foot and put a small insert into the other shoe though I am not a fan of such a solution. I put up with the difference.
Injuries can also lead to one foot bigger than the other depending on how we compensate for and heal from the injury.
Another possible reason that one foot is bigger than the other is the psoas muscle affects the foot even though it is located deep in the core. The classic pull of a tight psoas muscle is the shortened psoas pulls the affected leg tighter into the hips socket externally rotating it slightly. This external rotation translates into a lifting of the inner arch toward the lateral side of the foot.
When the psoas is tighter in this way it can pull the arch of the tight side foot up in the midfoot, pulling all of the bones of that foot closer to one another. In this case the shoe of the foot might fit poorly because the bones pulled closer together make the foot slightly shorter. The fit of the shoe is often less comfortable in the arch as the higher arch can press uncomfortably into the tongue of the shoe.