It's a turtle. Or at least that's what he looks like, and those usually don't grow any bigger than you can hold. But he might be growing beyond that, if the imps are to be believed—which they usually aren't.

[ In her experience. By which we mean that Katsa holds grudges and the imps earned them the day they brought her to the Drabwurld, so she's not going to let it go. ]

I hope he doesn't grow as large as a building, and that we all find ourselves menaced by a turtle...