Java Edition:Mob with Scale Attribute
In April Fools' snapshot 23w13a_or_b, you could temporarily change the size of a mob or player by using certain potions exclusive to that version. After upgrading the mob/player to a certain version range, it would permanently keep its discontinued size.
Obtaining
Firstly, you will need to obtain size potions (Potions of Big and Small), either in their potion, spash potion, or lingering potion forms. You then need to either drink the potion or apply it to another mob and you will notice that its size will change. You then need to Force Load the world containing the entity that has the big/small effect to at least 23w51a (1.20.5 snapshot), and the entity will permanently be bigger or smaller. This is because of the addition of the generic.scale attribute, which changes the size of entities.[1]