Why Heisei Godzilla anf its monsters caps out at continental at best

Comes from Godzilla's fight with Battra which during their fight they broke a tectonic plate in the process.

All this talk that Godzilla and co being star level because SpaceGodzilla was born in a black hole was all a theory by the human characters at best who were speculating SpaceGodzilla's origin. All we really know is either Biollante or Mothra unknowingly carried Godzilla's cells on them and mutated. For all we know his cells could have been mutating in its vacuum from from spaces radiation.

And Heisei Godzilla would have only been planet level in his burning form had the GDF failed to prevent Godzilla from blowing up taking the planet with him.