I hadn't seen the announcement that every fat binary in 10.5 had code for:
- Legacy PPC 32-bit
- Legacy PPC 64-bit (G5 only)
- x86 (for the Yonah systems)
- x64 (for Merom and Xeon systems)
Most of them don't. A large amount of code in Leopard has both PPC and Intel 32-bit code, but only some parts of the system also include 64-bit code (Apache is one example I've found). Some will be PPC-only or Intel-only, e.g. drivers for hardware which only exists on one processor family.
Or, maybe you should claim "6x", and split "Legacy PPC 32-bit" into G3 and G4 (AltiVec) systems and x64 into SSE4 (Penryn) and SSE3 systems.
There is almost no G3 support in Leopard at all. The oldest supported architecture for most code is "ppc7400" (G4). For example, Address Book contains G4 and i386 code. Only applications which can run on earlier systems (e.g. iTunes) are still built with G3-compatible code.
Certain applications which benefit from SSE4 might justify creating a separate code architecture, but I expect most will take advantage of it through common frameworks, or writing alternate versions of functions and calling the one optimised for SSE4 if appropriate.
There are only two Mac models released with Penryn so far (Mac Pro and MacBook Air), so I expect very little code prior to 10.5.2 will be written to take advantage of SSE4.
Look at the size of PPC vs Intel updates for 10.4: the Intel ones are generally in the order of double the size of the PPC ones. The larger updates are mainly due to containing two architectures for everything: 10.4 Intel updates include PPC code for Rosetta, 10.4 PPC updates are PPC only.
If an increasing number of files in Leopard have more than two architectures, this could account for a "more than doubling" effect, but I'd expect this to be a small proportion of the total.