That means “fully metallic” so if you’re unsure why the exports look metallic in three.js, there’s your answer. Or are you asking why Blender shows 1.0 to begin with?
And for now GLB is a total outsider in terms of file size.
It looks like the BabylonJS exporter has quadrupled the number of vertices in the original geometry, somehow. Maybe they’ve converted it from an indexed mesh to triangle soup… If I start with the FBX file and convert it using Facebook’s FBX2glTF, I get a much-more-reasonable 900kb with the same visual result. Enabling Draco compression in FBX2glTF, you’re down to just 100kb.