pure decoder width isn't enough to tell you everything. X86 has some commonly used ridiculously compact instructions (e.g. lea) that would turn into 2-3 instructions on most other architectures.
The whole ModRM addressing encoding (to which LEA is basically a front end) is actually really compact, and compilers have gotten frightently good at exploiting it. Just look at the disassembly for some non-trivial code sometime and see what it's doing.
