i have a theory: Given the duck's size, i can guess that the duck is gonna be very slow. If a duck is slow when it's a duck, then it's gonna be exponentially slower as it grows in size.
On the other hand, Given the horses size, i can guess that the horse is gonna be very fast. If a horse is fast when it's a hourse, then it's gonna be exponentially faster as it shrinks in size.
Having said that, it doesn't mean the the horses will cover more distance when they attack, it just means they attack very quickly.
Since horses are bright and can be trained to be organized, such horses can attack in different squadrons using different tactics. We can mount different weapons on the hourses and each one can act as it's own unit with its own targeting systems. While the duck has to control ALL the weapons at once.. therefore we need one hell of a very efficent network (no, it's not gonna happen... that network is very vulernable compared to such network between the hourses)
thefore, size might not matter. What matter is communication and independence of the attack. Since the horses can be modified in such way, i think the horses can be very deadly in such battle.
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