The raw material of cotton vest

is cotton mainly. Machines spin cotton and weave the cotton yarn into cotton cloth.

When being weaved,

the Cotton fibers will accept the external force and be stretched.

They will keep this state until they encounter water.

Water can reduce the friction between the cotton fibers and break this stable state.

In the meantime, the cotton fibers swell after water immersion.

Both of these two reasons leads to the shrinking of cotton vest.