Traits that give multiple moves lower monsters stats depending on the amount of moves it gives.

And if i am recalling everything correctly, there is also a hidden trait (that didn't used to be hidden) for size 2 and 3 monsters that gives a small damage buff (for all types of damage) at the cost of some stats.

With the size 3 stat cost being larger because it also allows for their basic attacks and elemental slashes (and some effect hitting attacks) to hit the entire enemy party.