Back in the day (2012), the Nokia 808 PureView had to manage with a custom ISP that did all the hard work of oversampling and real time zooming, with great performance, while the Lumia 1020 added OIS to proceedings but lost out in terms of speed, using the main Snapdragon S4 chipset to do the computations needed. The computing power (processor/GPU) in 2018 would have been unimaginable five years ago, but we've now reached the stage where three cameras can be used at the same time, one of which is a 40MP sensor, as per the quote below, and with the immense computing power handling the merging and interpolating needed.
As a result, the (up to) 3x zoom possible on the 808 and 1020 (depending on settings and set-up) may now be upped to 5x, thanks to fancy computational means. And all without needing a physically huge/deep camera unit. If Huawei pulls this off then the pixel size will be even smaller than that on the Lumia 1020, but sensors have improved so much and there's so much power available these days for image noise reduction, that it really shouldn't matter.
From the news article:
The triple camera has been the cornerstone of the Huawei P20 Pro marketing campaign, but concrete details have been scarce. The new camera, co-developed with Leica, may pack a 40MP sensor, the likes of which haven’t been seen since the PureView days. Here’s the breakdown.
The 40MP sensor will serve as the primary camera. Next to it is an 8MP telephoto camera with “hybrid zoom”. According to unconfirmed info this cam will provide 3x optical zoom, which along with info from the 40MP sensor will help achieve high-quality 5x magnification.
The third sensor will have 20MP resolution and will likely shoot in black and white as well as assist with bokeh effects. The triple camera will be assisted by Laser AF and an IR-RGB sensor. As for the slow-mo videos, the P20 will reportedly shoot 960fps at 720p (same as the Galaxy S9 phones).
Whatever next? Three cameras, OIS, massive processing power, laser autofocus, the Nokia pair now look somewhat 'simple' by comparison! They still compete, mind you, even in 2018, and I'm looking forward to pitching the Lumia 1020 in particular against an (alleged) new triple camera Huawei to see if the modern zoom implementation can match 'old faithful'!