Why is the 64-bit version of Windows bigger than the 32-bit version?

The difference in size for the Windows 8 Consumer Preview ISO images, between the 64 bit and the 32 versions is 800 Mb or a difference of 24%.

I've never seen a difference so large.

Why is there such a big difference?

Solution 1:

There are several reasons for the size discrepancy:

1. 64-bit Windows contains a translation layer (Windows 32-bit on Windows 64-bit, or "WoW64") that allows it to run 32-bit software.
2. 64-bit Windows also contains 32-bit versions of various programs (Internet Explorer) and libraries (in the WinSxS folder). As noted in the comments, this is the most significant reason for the size difference.
3. 64-bit memory addresses are twice as long as 32-bit memory addresses, so 64-bit pointers are also twice as long. 64-bit programs that make heavy use of pointers will be noticeably larger than their 32-bit counterparts.

The first and last reasons do not contribute nearly as much as the 32-bit software that ships with 64-bit Windows - but for the sake of completeness, they should be considered.

Solution 2:

In 64bit-versions, Microsoft deploys additional software. For example, there are two versions of Internet Explorer (64 and 32bit). Another example is the whole runtime system for Win32-applications.
Also, binary code will get bigger. Probably the sum of this these makes the big difference.