It's 15 million of codes the full source, that includes supports for like 10 different hardware architectures, and device drivers for every hardware that was ev

It's 15 million of codes the full source, that includes supports for like 10 different hardware architectures, and device drivers for every hardware that was ever made! When you compile the linux kernel you will compile only a small fraction of these lines of code, and distributions typically compile drivers and a lot of things as modules, so you load them only when you need them. You can compile the kernel to fit in as much as a couple of Mb of space, very useful for embedded systems (my openWRT router has a 8Mb of internal flash memory, and runs Linux 4.4!) And if we compare Linux with for example Windows is pretty small, Windows kernel source is said to weight a couple of gigabytes, and this for a kernel that supports only one (ok, now two with Windows for ARM) computer architectures and doesn't include device drivers (besides the basic ones)