I am known for making controversial titles like these. I'm also known for making rovers competent after complaining about them for years. But who cares? This isn't relevant.
There are big advantages for smaller armies that don't apply to big armies. A projectile may struggle to hit a single unit, but the projectile easily hits an army of units. A single unit does not need a varied army to be effective. An army will struggle without army-specific balancing units. It is easy to micro a small clump of units. It is hard to micro a big clump of units.
How is this relevant? Well, small armies being stronger is the reason it becomes so hard to attack into the opponent's army once it loses a lot of weight. Even if the big army crushes the smaller army anyway, it will take the big army more time to destroy a small army than it would take for the big army to destroy units from a bigger army.
For this reason, several small armies might even have higher power level than one big army. And for this reason, two teammates commanding two small armies may prove more effective than one ally commanding one army with the size of two teammates.
However, small armies being stronger also proves that strength is not always prioritized by "survival of the fittest." Even if a small army is more powerful, a critical mass of an army means that even if the critical mass loses some units, enough units deal damage to crumble the small army.
These all are just speculations and theory-crafting. Don't expect to use them in a game effectively. However, I hope you enjoyed my ramble.