See Can I apply DRACO Compression with GLTFExporter when exporting it? … THREE.GLTFExporter currently doesn’t support this. That’s a lot of work, and I can’t tell you how to do it sorry.
I would really suggest verifying why the file is large before you invest time in a particular way of optimizing it. A GLB file should not normally be larger than the FBX equivalent, I think something else is wrong here.