I have a bunch of z-wave, too. Z-wave and Zigbee, I think, only broadcast when needed. That's why they're much more battery friendly. If they're plugged in, they do more listening and re-broadcasting to do what you're talking about.

Wifi has, relatively, a ton more traffic. DHCP renewals, keep-alive messages, and basically always listening all the time to see if something is looking for it. That being said, these smart switches use very little data overall. Unifi shows them using about 700 bits per second, which is 0.0007 Mbps.