“a” — scrollbar thumb height,
“b” — your screen height (which equals scrollbar height),
“c” — displayed document height
a/b = b/c
I.e. your scrollbar thumb height compared to your screen height represents how much of the whole document is displayed on your screen. In other words, what fraction of the whole document is visible to you now.
The same with horizontal scrollbar.
Look at the scrollbar thumb size for this page: it’s height is about 30% (depends on device, actually) of your screen height — which means, that the whole article with comments is just 3 screens to scroll.
Now you know how to estimate any scrollable list size at a glance!
Simple, clear and useful.