The point of this thread is not to get "solutions", as there are myriads of ways to customize that, but really a "why" question.
Why are the most common GUIs so "big" by default, unless you are on at a least a 4K display? Default up to FullHD, as I said, is way too big, at least IMO and compared to other OSs.
So, I wonder why that is, as it's nothing new as I said. It's not just because most active Linux devs would have large high-DPI screens, as OTOH defaults usually do not work that well with high-DPI displays either. Maybe most of them have poor eyesight? I dunno.
Sure the fact that Gnome has migrated to a model heavily targeted at touchscreen mobile devices is one reason (even though I doubt that many users of tablets use Gnome...
)
But it tended to be like that even before mobile devices were a thing.
Maybe it has something to do with font rendering, precisely - as small antialiased fonts tend not to render very sharply on Linux compared to Windows or macOS, so they use larger defaults.
It indeed looks like this "big UI" trend started when antialiased graphics on Linux became the norm. I think there are ways to improve font rendering, but it's not quite great yet... just IMHO. YMMV.
And don't tell me to switch to non-antialiased fonts, I really don't like that. Some do, to each their own.