I think they're a bit on par on size. I have only seen photos of them side by side, thanks to living here.
I've heard - can't prove it though - that Viaggio seems to last a bit longer for some. I can probably PM them too you as well if you want. Haha.

But space wise for us, a rearward facing carseat meant no passenger almost haha. But the Viaggio was about 10+cms shorter.