In Xilinx's case, the toolchain is huge because it isn't just bitstream synthesis; it also includes:

* Simulation and verification tools

* Licensed IP cores for a bunch of common tasks

* Toolchains for at least three different platforms (ARM, Microblaze, PowerPC)

* 32-bit and 64-bit versions of everything

* A JRE

Bottom line is, there's legitimately a ton of stuff in there.