Android uses Linux as its kernel. But it is much more than just a kernel. It's all of userspace as well, including tools, applications, and so on. A proper comparison would be Android's source to the source of an entire Linux distribution, not just the kernel. The source code for Debian's latest release takes 8 DVDs or so.
Actually, Android also has to include the source needed to make the toolchain to support Android's target platforms. So you have the source code of various tools that don't even run under Android included in the distribution. The Linux kernel source requires you to already have a toolchain (compiler, linker, assembler, and so on) that supports your target platform.