Because the structure of the waist material is different. The fabric, lining and yarn direction of the waist are almost the same, so the waist is similar to the waist shape of the human body.
 

Although men's trousers take straight yarn from waist to waist, they use a slightly thin skew yarn, which is divided into two upper and lower ones, and the lining is also arranged in the waist. When waist is installed, the waist of the trousers is sewn up, which will make the male waistline form an arc. Moreover, there are differences in the hip and waist of men and women, and the size of the arc of the waist will also be different. Therefore, the waist of male trousers is slightly straight, and the waist of women's trousers is more curved.