Minecraft 1.20.5 introduced the Scale attribute which lets you change the size of mobs. This plugin gives the server owner the ability to vary the sizes of spawned mobs so that not all mobs look identical.
3 chickens spawned in various sizes, you can see their hitboxes are also resized
By default the size variations are set low but you can set the ranges to anything you want.
3 cows spawn in at different sizes