g++ is primarily the compiler and standard C++ library. VC++ is a full IDE, complete with the debugger, and many additional SDKs. (Win32, MFC, COM, DirectX (though not the lastest), etc.) It's been awhile since I installed 6.0, but if I remember right it also installs additional database access stuff like ODBC drivers, etc.
In short VC++ is more like the equivalent of a combination of gcc, g++, Anjuta, gdb, GTK+, qt, gtkmm, unixODBC, etc. All of those eventually add up to more disk space than g++ alone...