This phenomenon is commonly known as crotch popping, and jeans lovers must be used to it. Fat is not the weight, but the size of the thigh/hip, which is based on whether the pants can be propped up.

Under the condition of sufficient support, the material of cloth determines that the friction force is much larger than that of ordinary plain cloth. In addition, the crotch is the seam of many clothes (see figure), which is the place where the whole pants have the greatest friction and tension, and it is a natural result that they will break.