x86 is old and crufty; it supports three instruction sets and four paging modes; its long and successful history means that it needs the most kvm support code. There are two different virtualization extensions that kvm supports on x86 (Intel's VT and AMD's SVM). It is also the architecture that has been supported by kvm for the longest time. It is no surprise that it leads the pack by a significant amount.
ia64 is a newer architecture, but a quite complex one. The mechanism by which is supports virtualization, with a module loaded into the host kernel and a second module loaded into the guest address space, also adds complexity. So it comes in second, though far behind x86.
s390 is older (and probably far cruftier) than x86. But on the other hand, its hardware virtualization support is so mature and complete that a complete hypervisor fits in a fraction of the lines required for x86. Indeed, it will take a while until x86 can support 64-way guests.
ppc 44x, the embedded PowerPC variant targeted by kvm, has a simple software-managed tlb model, and the regular instruction set encoding favored by RISC processors, so it gets by with just a seventh of the amount of code required by x86.
As we add more features, kvm code size will continue to grow slowly, but the relative comparison will no doubt remain valid. And kvm will likely remain the smallest full virtualization solution available.