x86-64 somehow manages to be less compact than ARM while still being variable-length. Thumb is still much saner than x86/x86-64.
It's certainly possible to design a CISC ISA (or maybe even a RISC one) that's optimized for compactness. x86 isn't that, though. Maybe it was in the 16-bit era, in a hypothetical future where BCD instructions were used a lot.