It's true that Iz*One had a way bigger fandom and better album sales but i wouldn't say they were bigger than IOI, if you were around 2016 u know none of Iz*One songs are as big as Very Very Very and Downpour. They didn't really have a huge hit. IVE's songs are bigger than any Iz*One song ever was.
I'd say the agencies are promoting these kids better than they did with IOI members, and the 3rd generation was already saturated with top groups from the same generation back in time: Twice, Black pink, Gfriend, Mamamoo , RV were all very big when these ggs debuted. As for 4th generation, barely anyone was yet established as a certified top group. Foreign members help so much with foreign fans as well.