You can see the difference between our default and compact noteheads for notes positioned between stave lines when zoomed in. The compact sits exactly on the center of the lines, whereas the default notehead covers the thickness of the stave lines, which is generally a more pleasant look.
If you want smaller noteheads for semantic reasons, you’ll have to convert the notes to gracenotes. There is further control over sizing in Dorico, which we don’t currently provide in Cubase.
What’s your specific use case of requiring smaller noteheads?