Increasing max body shape limits is part of game progression, connected to purple orbs + the mirror in your room, plus some permanent size boosts for completing achievements.
But I'm pretty sure its meant to be a gradual thing either way, like as long as your size is either bigger or smaller than what you went to sleep as, you're gonna advance one step in either direction when waking up, at least from what I remember. I don't think anything else can come with you, the game is more focused on bringing stuff into the dreams than out of them.