Jahy-sama wa Kujikenai!

Jahy used to be the Dark Realm's feared second-in-command, surrounded by power, servants, and an enormous mana crystal. Then a magical girl shattered that crystal, destroying the realm and throwing Jahy into the human world with most of her magic gone. Her imposing adult form now lasts only when she can gather enough mana; otherwise she is stuck in the body of a small child living in a cramped apartment. Restoring the Dark Realm requires recovering the scattered crystal fragments, but first Jahy has to survive rent, part-time work, hunger, and a landlady who is unimpressed by former demonic status. Her pride insists that her fall is temporary, even as ordinary human life keeps defeating her in increasingly embarrassing ways.