[Kelberon]

Hmm.

Well, if Mattie's Ego is about being nurturing, maybe that should be the reason it happens in this particular case-for example, perhaps Otome has endured a rough day, so when Mattie sees her, she's very upset. This triggers Mattie's Ego, or causes her to drink some milk and trigger it, with the idea of trying to calm Otome down.

Of course, if that's too serious, you could end it on a silly note, like Otome bouncing back to happiness before Mattie does anything, leaving Mattie in her Ego form for the next few minutes/hours for nothing.

I know you've probably had ideas like this already, but I figured I'd try something that wasn't just "Oh! Breasts got bigger!" I always like to have more of a story, as long as it's not overly elaborate.