Win X64 says 399MB
Lots of stuff is included. You may not use all the features (now

), but it has support for MANY devices and formats. News Sources...
It also includes fine tuned software libraries for Calibres private use, rather than fight with all the (lame) variations out there.
Note Windows installs some of the same (named) DLLS with every program
(eg MSVCRT80 )