That doesn't mean they're the same diameter, it's just as wide and has as an equally sized sidewall. See
here (1010Tires.com) and
here (tire calculator). So it looks like both the tires and wheels will be heavier on the Cobalt SS.
Anyway, there's a lot more to the rotational mass/unsprung weight issue I'm not qualified to answer. I'm no physicist/engineer (although my work title claims I am an engineer. hehe). It's very possible that a 20lb 16" wheel will spin much more easily (and faster) than a 17lb 21" wheel.
You've got to make a compromise/sacrifice somewhere. Looks vs performance. But I see a problem. I think downsizing the wheel size of the Cobalt SS while keeping OEM diameter will make the tires look crazy fat. It may also hurt handling because your sidewall will allow for more flex. Not using OEM tire circumference will throw off your speedometer/odometer readings.
If you want your speedometer/odometer to remain correct, one possible solution I can see is using the ION RL's OEM tire size and (if interchangability allows) swap out the Cobalt's
vehicle speed sensor for the one in the ION RL. The difference may be as simple as a gear used to adjust for the tire size. But sometimes these things are in your differential and you may need to crack open the tranny for this. Keep in mind this is all speculation. This should not be attempted until someone confirms this is possible.
From all the reviews I've seen (not always the best source but anyway), they believe the Cobalt SS is slower than the Ion RL by ~0.2 sec.