Yes, force multiplication (or division) would be the way to go.
We cannot exactly lie about things that are notified in the pop-ups, e.g. founding religions or world wonders, or they'll catch on pretty quick.
Deception about our levels of expansion wouldn't work terribly well, either, as it would take just one explorer to prove us wrong..
But, our forces are mobile, and not neccessarily confined to our cities or even our lands.. Even with a strong spy network, they would have trouble keeping track of our numbers if we moved each unit around every few turns.