im thinking either:
1) she realised that if she is bored of enorme, she can just transform her into her ideal opponent, a magical girl.
or 2), she has been younger this whole time, and part of her transformation is getting taller and older, as part of her idea of a "strong ruler".
i prefer the first one because even though enorme is tall, she's still a "girl"? the idea that enorme looked too old to be a magical girl is whack imo