I'm pretty sure Bagley's wingspan is 7'0" but not the point, that 4 inch difference is massive. But it's more than that, Bagley's standing reach is 8'9" which is not a great sign at all. We don't know Giannis's exact measurements because they've never been exact and he has grown significantly since entering the league but if you watch him play and see him dunk while barely even jumping it's pretty clear he is a hell of a lot longer than Bagley.
Even with the most generous measurements there is absolutely no way that Bagley can do this:
https://twitter.com/ESPNNBA/status/853380979854946304?s=19
His standing reach has to be at the very least 9'2" based off of this, that's just a massive difference than Marvin Bagley.