I've only done custom kernels on 32bit x86 headless server
platforms, so maybe mine would double in size if I were doing X86-84 kernels due to 8 byte instruction words vs 4 byte words,
I expect that this is merely terminology, but I don't think that 64-bit x86_64/amd64 architecture exactly has eight byte instruction words; rather it has 64/128-bit registers. What mainly changed is the addition of many more general purpose registers, and the support for 64-addresses (i.e. pointers.)
You are correct that virtually all 64-bit executable text binaries are much larger as a result.
Mark Allums