Linux or any other Unix has spoiled its user with too many desktop environments and window managers. So maximising the Emacs frame through an Emacs command is not done for those systems.
But since Windows is not so,
w32-send-sys-command seems to do the ‘maximise’ window of the Emacs frame properly.
I don’t use Windows at all, leave alone Emacs on it. But who knows? I keep this tip for the future 😉