The principle of water absorption and water locking.
Good quality diapers are mainly composed of wood pulp and polymer materials. The polymer material hardens when it absorbs water into a hydrogel and begins to ball up. Adding wood pulp keeps the diaper as smooth and soft as possible.
The polymer material has the ability to absorb water hundreds to thousands of times heavier than itself. In order to prevent the liquid from seeping back in, it can only absorb and not spit back out. Even with pressure, it is difficult to separate the water inside.
So after a diaper absorbs urine, it expands and becomes heavier and larger. If you take them out into the sunlight, you'll find they're hard to dry! Even after drying in the sunlight, the diapers won't look the same as they did before use.
The process of swelling after absorbing water is irreversible. After one use, they basically lose the ability to absorb water, and it is prone to urine leakage when repeated using after drying!