Bloom War!

Two civilizations use size as a weapon in an all-out war. A race of humans that can absorb each other's size to grow themselves. Futanari are very common. There are three warring factions that are constantly using size as a weapon on the battlefield. There are only a few ways to steal someone's size. One of the few natural ways is being able to insert someone into yourself and absorb their "pollen". (Putting a cock in someone doesn't count.) The most common way someone would steal another's size is by inserting them into their pussy. There are artificial ways too. Potions, artifacts, magic, machines, you name it. There is also a rare breed of human known as a stamen. Stamens cause anyone who takes in their sexual fluids to grow and are desired for the wars, obviously.