When playing Supremacy 1914 you want to be smart about army composition and how to group them into armies. One major reason for this is the size factor feature.
When an army reaches a certain size, each additional unit of the same type added to the stack only adds a fraction of its damage to the total damage of the army. Size factors are written in this fashion:
a0=1, a25=0.5, a50=0
The above example means that starting at 0 units each additional unit adds 100% of its damage. But starting at 25 units each additional unit only adds 50% of its damage. Finally, starting at 50 units each additional unit adds 0% of its damage to the total damage of the army. This is why splitting up armies often is a good strategy for maximizing damage output.
Here's an overview about all units and their respective size factors: