The flippant answer is: "Because old APIs never die, we just add new ones."
The biggest single part of the SDK is the documentaion. The biggest part of the documentation is the WinFX API reference.
Below is a breakdown of the January CTP:
Win32: 211 MB (reference and conceptual)
WinFX: 467 MB
- Conceptual/Tools/GenRef/Portals: 47.3 MB
- Integrated Samples: 103 MB
- Managed Ref: 253 MB
- Collection-level files: 63.7 MB
As you can see, the managed API reference pages alone are over 250 MB (that's in a compressed help file). These represent 80 assemblies with 281 namespaces and 10741 types with 103704 members. Each one has a reference page.
Now consider that Microsoft has been working on Win32 styles APIs for much longer than WinFX style APIs. There are over 72000 reference pages in the Win32 documentation. Counting the number of Win32 APIs is trickier--it's even more challenging to count COM interfaces.
The other key parts of the Windows SDK include the tools. In the January CTP we ship a bunch of C++ compilers and other tools.
Last but not least, are the samples that exist only in the file system--most of the Win32 samples fall into this bucket. Luckily, these compress very well for download.