Cloth is woven in 90 or 150 cm widths. Square-makers have to balance between fitting in the pocket and wasting expensive silk or linen. Usually the cost of the cloth wins.

Industry standard for a handkerchief is 16-19" on a side. What is called a pocket square is usually 11-12". Folded or stuffed, the 11-12" sizes tend to fall down and disappear in a pocket.

Handkerchiefs may be too large to fold but are right-sized for stuffing, and stuffing is what you want to do anyway.