RISC doesn't imply wasted instruction space; RISC-V has a particularly interesting thing for this - with the compressed ('c') extension you get 16-bit instructions (which you can determine by just checking two bits), but without it you can still save 6% of icache silicon via only storing 30 bits per instruction, the remaining two being always-1 for non-compressed instructions.
Also, x86 isn't even that efficient in its variable-length instructions - some half of them contain the byte 0x0F, representing an "oh no, we're low on single-byte instructions, prefix new things with 0F". On top of that, general-purpose instructions on 64-bit registers have a prefix byte with 4 fixed bits. The VEX prefix (all AVX1/2 instructions) has 7 fixed bits. EVEX (all AVX-512 instructions) is a full fixed byte.
ARM64 instructions are 4 bytes. x86 instructions in real-world code average 4.25 bytes. ARM64 gets closer to x86 code size as it adds new instructions to replace common instruction sequences.
RISC-V has 2-byte and 4-byte instructions and averages very close to 3-bytes. Despite this, the original compressed code was only around 15% more dense than x86. The addition of the B (bitwise) extensions and Zcb have increased that advantage by quite a lot. As other extensions get added, I'd expect to see this lead increase over time.
x86-64 wastes enough of its address space that arm64 is typically smaller in practice. The RISC-V folks pointed this out a decade ago - geomean across their SPEC suite, x86 is 7.3% larger binary size than arm64.
So there’s another small factor leaning against x86 - inferior code density means they get less out of their icache than ARM64 due to their ISA design (legacy cruft). And ARM64 often has larger icaches anyway - M1 is 6x the icache of zen4 iirc, and they get more out of it with better code density.
Also, x86 isn't even that efficient in its variable-length instructions - some half of them contain the byte 0x0F, representing an "oh no, we're low on single-byte instructions, prefix new things with 0F". On top of that, general-purpose instructions on 64-bit registers have a prefix byte with 4 fixed bits. The VEX prefix (all AVX1/2 instructions) has 7 fixed bits. EVEX (all AVX-512 instructions) is a full fixed byte.