CMG might make $12M for UFC 194 --- that would be largest payday in UFC history (estimate for Rousey's last fight is $5-10M).
Right now, Rousey is the bigger star. But we'll see in 6-9 months. Half of Rousey's appeal/mystique was her undefeated status.
Meanwhile, McGregor's stock is only going up, up, up. With a couple more wins, he could become the edgy/trash-talking international version of Peyton Manning.
I'd also say McGregor is bigger than Couture, GSP, Liddell --- only because the sport has grown so much in the last 10 years, 5 years (especially worldwide). Did ESPN cover UFC in 2005? Nope, not really.