The term “volume” is sometimes in a general way to refer to the volume of the front part of a foot.
There is no particular standard for measuring foot volume, and foot volume is rarely if ever stated in number units. Rather, a foot with relatively greater foot width and instep height, with correspondingly large ball girth, arch girth, or instep girth is said to be “high volume”, or the opposite kind of foot “low-volume”.
Particular kinds of footwear, such as ski boots, are sometimes sold in low, medium, and high-volume sizes, rather than in different shoe widths.