This is a seriously scary fate. Not that Mandy is a giant, that's comprehensible. But the fact she's outgrowing the universe and is just stuck in an abyss of a void forever is incomprehensibly terrifying.
For an idiot, Billy has a point in this episode when he refuses to change Mandy back. Like he said, Grim always helps Mandy get her way, so Billy’s not going to encourage her.
You should do another one like this! There's that episode where Grim takes Billy & Irwin to Japan and Grim enlarges Billy to help fight a Giant Monster.