acid is sometimes denser than water
if a small quantity of water is added to excessive strong acid the water would mostly float on the acid layer , as the hydration of acid ions would release a lot of heat violently , causing the small amount of liquid on top to potentially boil and splatter, whereas when adding a small quantity of acid to water the acid droplets would sink and travel across the water layer , reacting with surrounding water in a similar way....however since the energy is more evenly distributed the reaction wouldn't be as violent.
so if katsuyu spread excessive acid to sucessfully cover up suigetsu he's done while suigetsu can effectively cause boiling to occur if he spreads small quantites of water all over katsuyu's acid b4 it is launched