According to the Ochre Jelly's entry, slashing damage splits it into two smaller jellies with half hp
Split: When a jelly that is Medium or larger is subjected to lightning or slashing damage, it splits into two new jellies if it has at least 10 hit points. Each new jelly has hit points equal to half the original jelly's, rounded down. New jellies are one size smaller than the original jelly.
I've been looking through the rules and can't find if there's any connection between damage and size, other than that small creatures can't use heavy weapons or that large creatures use two handed in one hand (I think).
Are there any rules regarding size reduction's effects on weapon usage and damage?