In a future world, how large would Earth-based militaries be?

My world has roughly the same technology as The Expanse, more advanced in some ways and less in others. As such, each of the world powers have the technology to destroy each other many times over again and keep somewhat large fleets of combat spacecraft. With this in mind, would there be any need to have any conventional forces on the ground? Like tanks, APCs, amphibious assault craft, helicopters/VTOLs, etc. Since the powers obviously don’t want a nuclear conflict, would they still keep millions of ground troops in case of a conflict?