I definitely would not say it is a sizable portion. I'd say it is infact a minority of Youtube's audience. Their CEO revealed they have 1.8b logged in users [1] every month watching videos.
>Their CEO revealed they have 1.8b logged in users
That's what you get when you are logged in in gmail at the same time. It's not like people WANT to log in on Youtube, it's just a side effect of a google account login on some other service.
This isn't a huge percentage of the time for me, but I often use a device that isn't logged into anything because it's using the android builtin browser, someone elses computer, etc. Not having to care about logging in is a benefit.
That's what you get when you are logged in in gmail at the same time. It's not like people WANT to log in on Youtube, it's just a side effect of a google account login on some other service.