That's because the browser had to do things the sneaky way.
Everybody wants to control the app platform. Every time a cross platform solutions appears everyone tries to shut it down. See: Java, Flash, etc.
The only one that platform owners couldn't really shut down (though they are getting better at it), is the web. It was considered too "dumb" to shut down and too useful to completely avoid. And bit by bit, just like the boiling frog story, people added interactivity features until we arrived where we are now. A bloated, perhaps crippled cross platform solution, but the best we have now for running things on things from embedded, mobile, desktop, etc., etc.
Interesting metric actually. Building Windows takes 12 hours [1]. It's a bit harder to find metrics on Linux. You can build the kernel in 60 seconds apparently [2] but that is not a complete operating system.
That said, core-image-sato used to be just a simple demo image. In my old build system, we would build the Arago Project for a TI SoC every night and it would take a few hours but that included a lot of the DSP code as well which was really slow too. So a couple of hours on average is my guess.
Building my Linux distribution from scratch takes about an hour on a modern system from top-level ./configure && make -j5 (which produces the installer ISO image), a great deal of the time, however, is spent in first building the cross-compiler toolchain (to ensure that no matter where you build my Linux distribution you get the same results).
This is the cross-compiler toolchain, the kernel, and about 250 external packages.
Can build base+X11 BSD systems in a handful of minutes on modern hardware in one command. I don't have modern hardware, and build over NFS, so I can't give a precise figure.
But yes, windows includes lots of API's and frameworks and GUI apps and such. Probably something comparable complexity wise would be Base+X11+(1 of KDE or Gnome)..
How do you define the OS? If it's just the kernel, sure, but then is it really surprising? If it's also all the userspace necessary to e.g. run said browser, then I don't think this will be true anymore.
"We've come to the point where building the browser from scratch takes more than building the OS itself."
It doesn't. It took ~5h to build a tiny (~150 MB) complete system (Linux kernel + Yocto based "OS") from sources on 4 core PC few years ago. With modern CPU, I guess, it might be 3h or so. On the side note, the build process generated >50GB of files.
I admit I have not done so in a while, but I vaguely remember building Net/OpenBSD from source was faster than that on moederately powerful hardware (Core2 Quad 2.4 GHz, 8GB RAM, no SSD).
I think NetBSD took ~2.5 hours, including building its toolchain.
I am pretty sure building a common set of packages for, say, Fedora, or any version of Windows takes a couple orders of magnitude more resources than building Chrome.
On my Macs I've been doing "port install -s" most of the time and I can tell you some relatively small installs can take a very long time.
