Mob Variety
This datapack utilizes the new scaling attribute introduced in snapshot 23w51a (and is therefore only compatible with versions starting from that one) to give every mob in the world slight size variation. The range spans from 80% to 120% of the mob's original size!
Configuration
You can configure the minimum and maximum size that mobs can have when spawning. Execute the commands /scoreboard players set %min_size mob_variety.config <size> or /scoreboard players set %max_size mob_variety.config <size> respectively.
Here
After making changes, you need to run /reload for them to take effect.
IMPORTANT: While the datapack itself shouldn't significantly impact performance, having oversized mobs in your world might reduce it, so be cautious when adjusting configuration values.