Your scene is far from empty (couldn't be any other way), there are plenty of objects there stored inside groups.
Try switching your outliner to Group view and you will find plenty of group definitions you can insert into your scene, with lots of high poly objects stored in them.
Not sure exactly how they are stored or where they come from, but they are causing your big file size. Will try to investigate further.
If you go into the Outliner > Datablocks view you can also see lots of data stored inside your file, like images, objects, groups, brushes, etc.
To actually delete data, select the (un)desired groups in the Outliner under Group mode, Right Click over the group name (not the icon) and choose Unlink option.
You have to save, quit file, close Blender and then reopen the file again, for actual data to be gone. Save again and file size should be reduced.
Ok, only way I figured out how to actually reduce file was to go into the Outliner, after unlinking all groups as described above, switch to Blender File mode, go into the Mesh category and manually select everything and erase it with Right Click > Delete. This reduced file size to about 5Mb
Due to the large amount it is probably easier to just select everything with A and deselect all other categories before erasing