UPDATE: After the usual purging, check for un-needed hidden geometry, removal of lurking layers, etc the file size is basically unaffected by removal of half the geometry...
...but if one copies the remaining (that is, desired)geometry into a fresh, empty iteration of sketchup and saves -that-, one winds up with a properly-sized file interms of what's been cut (that is, a 64MB file rather than a 128MB file containing the identical geometry.
Still, given that the fbx original was only 23MB to start with, the resulting 64MB file still seems excessively large (though at least it's now useful for something).
Does anyone know if skp files are just always inherently larger than fbx files describing the same object?