I downloaded the entire Linux source and observed that it is only 500MB. But when I downloaded the Android source code using repo sync it is more than 15GB.
I thought android was based on Linux and it added some more features. But are the features added so much so that it becomes 15GB?
What is the reason behind this?
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.
