Oppai Chiisakute Nani ga Warui no sa!

The protagonist, Miura Yamato, lives in Munyu Town—a place where, for some reason, all the girls are flat-chested. And not just flat-chested, but petite in every way.
Growing up in such a town, Yamato developed a deep longing for big breasts. He dreamed of burying his face in them, squeezing them, licking them, sucking them…
Driven by this obsession, he creates a new drug that promises to make breasts grow. He offers it to the flat-chested girls in town, convincing them to help him achieve his dream.
And since the girls are also troubled by their small chests, they agree to cooperate…

[Translated from dlsite]