^^^^^^
This!
The critical issue is not mm vs. in., but differentiating diagonal length vs. display area.
In addition, as several people have noted, Apple's Watch measurements, 41mm and 45mm, refer to the
vertical height of the watch —
not the diagonal or the display area!
Indeed, the Series 7 Watch, with only a 1mm increase in height ended up with a whopping 20% increase in display area over the Series 6!
The article here on the Watch 8 keeps referring to the diagonal as a “display size”, which is incorrect or, at least, misleading. For example,
”A 1.99-inch Apple Watch display size compares to 1.691-inches on the 41mm
Apple Watch Series 7 and 1.901-inches on the 45mm Apple Watch Series 7.”
Square inches or square millimeters are called for when discussing *area*. And, as with the Watch 7, the size and area of the bezels needs accounting for, too!
The entire business of referring to the diagonal of a device to represent display area can be quite misleading. After all, a standard #2 pencil (e.g., the classic Dixon Ticonderoga #2 is 7” long and has a diagonal of nearly 7”! But it certainly wouldn't have a screen area equivalent to that of phones with a 7” diagonal screen.
The same would be said of the Apple Pencil. If it had a screen, its diagonal would hardly reflects its display area!
The diagonal is only a useful proxy if the aspect ratios are the same or similar. However, as in the case of Apple Watch 7, the size and area of the bezels matter, too!
I agree with the calculation here that a 5% increase in the diagonal of the Apple Watch, assuming the same aspect ratio and bezel size, would translate into a 9-10% increase in area.
What is staggering is what that means for comparing the Watch 8 to Watch 6. To keep things simple, let's say that a Watch 6 has a display area of 10 (square whatevers). With that as the base, then the Watch 7 has a display area of 12 (20% more). So, a Watch 8 would have about 13.2 (10% more than the 7).
Overall, that means the Watch 8 would have more than a 30% increase in display area over the 6!