Question

when cotton soak water volume decrease or increase​

Answer 1

The most effective bath towels are made of 100 percent cotton because cotton is most efficient at absorbing or soaking up water. Cotton is able to absorb up to 27 times its weight in liquid water, according to Cotton Inc. Cotton's absorbency is also useful in what is known as “recreational performance apparel"--clothes used in jogging, exercise and sports. Cotton's absorbent properties occur due to a number of reasons, including its specific molecular structure and the structure of water.

Answer 2

Volume increases when cotton is soaked in water practically ! It have a high absorption rate
Just like when you soak a sponge in a water bucket
It gets heavier means it has increased its volume