This is the circumference of your feet, measured at the widest point, the ball.
When you go down in size (while keeping the width constant), the length of a shoe decreases. Additionally, all other dimensions decrease, including the ball girth. The widest part of the shoes also moves closer to the heel.
When you go up in width (while keeping the size constant), only the ball girth and width increases. (On combination lasts, the heel width increases, too.)