Hi!
There’s a lot of breast and ass expansion in the game — it’s actually one of the core themes.
Succubi drain the hero to grow stronger… which means their bodies (especially breasts and ass) grow as a result.
Viewing post in Kilgor & the Succubus World comments
The game shows the expansion during the Game Over scenes — so when you lose, you’ll witness the succubus growing as part of the ending.
However, some characters also experience permanent expansion during the fight itself, which continues all the way until the Game Over (where the final transformation happens).