Rainbow lets a tiny Spike explore every inch of her body before letting Fluttershy join in on the fun.

Spike is an adult in this fic, though whatever age constitutes an adult dragon is beyond me—longer lifespans and all that. For the sake of avoiding pedophile, let us say, in pony terms, he is over the age of eighteen, and would have been over six feet were it not for the spell. This means it's okay for him to explore a giant Rainbow Dash, or at least, the thingy in my pants thinks that's okay.