> mov rsi, file_load_va + message: [...] This instruction ends up taking 10 bytes: two bytes for the opcode, and a full 8 bytes for the address. We can replace this with mov esi, file_load_va + message to save 5 bytes

Nasm should perform this optimization automatically.

  $ cat t.s
  bits 64
  mov rsi, 0xffffff
  $ nasm -o t.o t.s
  $ ndisasm -b 64 t.o
  00000000  BEFFFFFF00        mov esi,0xffffff
  $ nasm --version
  NASM version 2.15.05 compiled on Sep 24 2020