And, if 5e groups are trending larger than before, (if they even are), what about 5e makes it easier to play with large groups than, say, 3e or 4e?
In my case, the group isn't trending any larger than before when we ran 3e. The gaming group is the gaming group and it has shifted in size pretty much independent of the edition of the game we play.
That said, 5e is substantially easier than either 3e or 4e because we depart from the strict grid. With a grid, movement space and character space become a big deal and larger numbers of PCs (and opponents) complicate the encounters when push comes to shove and swords are drawn. With theater of the mind, everyone's been much more flexible in placement and movement. We get through people's turns faster, and the game plays a lot more smoothly.
I actually just crunched a bunch of data on session attendance. We average out to four people per session. That being said, like many others, attendance seems to vary wildly (which frustrates me to no end). I've had sessions of seven people, and of just one person. It can be exceedingly frustrating. In theory, there are currently eight people in our gaming group.