Considering the average adult male thumb length is ~70mm and female is ~63mm[0], that would be 7-8% of thumb length increased in height and 4-5% in width.
That's not even including the very different screen interactions on XS vs the iPhone 8, where the screen itself was significantly smaller.
But the iPhone 8 is already 15mm taller and 9mm wider than the iPhone 5/5S/SE which is probably the most comfortable size for a phone. Plus, the fact that the screen itself is much larger makes it more difficult to reach everything with one hand.
I previously had a Note 8 for work and have just gone back to my Galaxy S7 which, while still big, is much nicer to use - even though I thought I liked the Note while I had it.
That is with 4.0mm Bezel on every sides. So If Apple shrink the Bezel by 1.5mm, It would have the same width. Assuming you don't do Curved around edges like Samsung, Huawei has managed to make the left right Bezel shrink to 2.1mm.
The major difference between X/XS and 8 is the weight. And this make the whole phone felt very different.
Slightly. You can feel and see the difference in size, but only if you hold them both side by side (as opposed to the weight, which is quite obviously different).
I noticed the size difference immediately when I got my iPhone X, and I still notice it every day, even if I'm not comparing it side by side with an iPhone 8. Maybe you got used to it, but I haven't, and likely won't.
I'm not you, so I can't confirm what you are experiencing, but have you considered that the screen is throwing off your perception of the phone's size? For me, the biggest difference has been in interacting with the phone, which means that my fingers have to actually cross the entire height of the device rather than just the area that used to be the screen.
You mean, like iPhone XS?