18+ Giant Ojousama

Oriko Fujidou is the buxom young head of the Fujidou Zaibatsu, a group that has a monopoly on Japan's vending machine industry. And serving at her side is her personal butler, the eccentric Dr. Sebastion. When a monster suddenly attacks the city, Dr. Sebastion slips his ojou-sama an experimental energy drink his team in R&D has been developing. As a result, she quickly finds herself growing into a giantess, matching the attacking kaiju in size! Will she be able to stop his rampage, or at the very least, keep her dignity intact?

(Source: MangaDex)