There is two kinds of battery widely applied in power bank: lithium battery 18650 and li-polymer battery. Power bank with li-polymer battery is thinner than power bank with 18650 battery since 18650 lithium battery is cylinder style. Normally li-polymer battery is in cuboid. And it can be produced in any size.
Difference between li-polymer battery and lithium battery.
Safer: Li-polymer battery would just be intumescent instead of explosion when short circuit happens.So li-polymer battery is safer than lithium battery.
Longer life span
Thinner: li-polymer battery is much more ultrathin than lithium battery.
Taking power banks with different battery for example, same capacity different size and thickness: