From Zac's feature:
I don't know about you, but there was just something special about the Lumia 650 that was released. It was a charming little device, with a surprisingly nice design and build quality, relatively low-price, and way better than deserved display. While the Lumia 650 XL has a bigger display and bigger battery, it just doesn't feel special.
I think it's the less-than-stellar display that's making this phone unspecial. The best thing about the Lumia 650 was arguably its display, and that magic is gone on the Lumia 650 XL. I imagine the choice to go for an LCD on the larger one was a cost-cutting measure, as OLED screens are somewhat more expensive the bigger they are.
But because of this cost-cutting measure, the Lumia 650 XL has no charm. It doesn't have a standout feature that would've put it above the rest at the time. If the Lumia 650 and Lumia 650 XL had launched together, the smaller one would've had a better screen and overall better experience than the larger one. It would've made no sense.
The display tech used, the port positioning (charging top middle!), both suggest that a final 650 XL, which might have been launched around the summer of 2016, might have been significantly different than the prototype here. Still, this is all an interesting insight into what goes on behind the scenes in terms of creating a smartphone and running through various protoypes.
Well done to Zac and his sources for getting hold of this, anyway. I wonder what else he can unearth from the Microsoft hardware stable before Nadella (short-sightedly IMHO) shut down Lumia production 18 months ago?