Reducing solute particle size increases the surface area of the particles.
Water molecules bond to particles of the solute when the solute dissolves in water. This process releases energy and help to break the solute apart. However, water molecules attack the solute only if the two are in contact, which happens only at the outer surface of the solute.
Smaller particle size means greater surface area. Reducing the particle size of the solute would expose more of the particles to water. Doing so promotes the contact between water molecules and the solute. More interactions would happen at the same time. It would take less time to dissolve the same amount of solute with smaller particle sizes.
