Although very little has changed outside of his physical assets, Spike feels a bit more comfortable being handled by Jack today than he usually does. Something about the difference in size being more expected, or maybe heteronormative gender roles that sit too deeply in his bones to be fully ignored.
Either way he's melting into the touch, arms wrapping around Jack's shoulders and hips pressing in with a slow roll, eager to see how much the novelty of it all is affecting Jack. Sure has in the past.