As someone who works in a software development department of an IT company, the "few programmers working on one product" sounds realistic to me.
However, remember that Microsoft has a massive amount of employees (even their Schiphol (Netherlands) branch has a shitton of people, and when I asked for the amount, even the employees themselves didn't know).
As when it comes to Linux, that's not necessarily true.
The kernel is being programmed by tens of thousands of volunteer programmers from all over the world at the same time, supervised by Linus Torvalds (who already
once warned that the kernel is bloated).
Anything the user adds to it has impact as well.
If you want to have a full desktop environment, but you have an old PC, you might choose to work with LXDE or XFCE for example.
And users with a more high end PC might pick Gnome, KDE, or Deepin instead.
And of course there are many more things that might impact to the complete picture, but kernel, window manager, and desktop environment are really important here.