How to stretch out/expand a backpack and straps?

i just got this really cute backpack, but it's 12" by 5" by 16". i'm taking all honors and AP cles next year, so i'm gonna be carrying lots of books in there. also, i stuffed heavy books in my backpack and tried it on. it looked like i was lugging a gigantic cube. i've been using my previous backpack for 3 years, but one of my straps broke (i tried sewing it on and it broke again) so i've given hope on that and got this new one. because i get back pains easily, i've increased the length of my straps so the backpack is lowered more to the waist/ area. i've compared the length of the straps of my new one to the old one. they're the same length. however when i scroll down the straps to its maximum, it doesnt go down as much and you know how the straps are made with the felt stuffed with cotton whatever and then its just this thick piece of string thing? well when i stroll down my new backpack straps so my new backpack would reach my waist area, the felt-cotton part of the straps only touch my shoulders! with my old backpack the felt cotton part would still reach my chest area. how can i expand my backpack (it's 12x5x16 now) and stretch out so i can lengthen my straps?