I suppose there is such a thing as stack bonuses in this world, or something like it, but it's abstracted into the background.
Yes. It's called bureaucracy. Units, especially COMBAT units, can only get so big before the chains of command and communication become... difficult. Active combat units need a level of clarity and efficiency in their orders so that they can function effectively.
Single units acting alone receive no bonus. Two units acting in coordination can attack with a force greater than the sum of the parts. You can add units to achieve a greater effect, but ONLY to a certain level before the amount of time and effort it takes to coordinate those units becomes too much and it can't be effectively achieved.
That's pretty booping close to a stack bonus mechanic.
"All warfare is based on deception" - Sun Tzu, Chapter 1, Line 18, The Art of War
"The principle of strategy is to know ten thousand things by having one thing." - Miyamoto Musashi, The Book of Earth, Go Rin No Sho