Question

why are the straps of the bags area larger

Answer 1

We know that there is application of pressure that is if the contact area is more then pressure is less. If contact area is less than pressure is more.
If the area of the bag straps is less then pressure is more it is difficult to carry heavy books because area is less then pressure is more.
Now if the area is more pressure is less now we can carry heavy books easily because area is more then pressure is less

Answer 2

because some people is long hight and some small so bags straps is lager you can feel Good and comfortable so bags area larger and I hope you understand