Items, which falls within the 2-piece carry-on allowance, include: Carry-on bags or suitcases (wheels and handles included in the size), briefcases, Laptop computers, diaper bags, camera cases, cartons or other similar items.
** Include all airport and duty free purchases in the carry-on allowance.
(standard article 23cm x 40cm x55cm - Personal article 16cm x 33cm x 43cm)
I use a smaller backpack which is just from my neck to my butt.
It doesn't look too big. But it can carry 15KGs heavy stuffs, like books.
No problem so far.
You want to put small and heavy non-metal stuffs in your backpack. Not something light and bulky, like cloth. It's allowed to carry 7-8 KBs in carry-on luggage.
I made 15KGs twice anyway.