Why is it difficult to hold a school bag having a strap made of a thin and strong string?

We know that lesser is the surface area of an object the more is the pressure it exerts. Hence, it is difficult to hold a school bag having a strap made of thin and strong string, because the thin string has very less contacting area in case of a school bag having a strap made of thin and strong string which increases pressure which is un comfortable to carry the school bag.