When your mac has a 15,4 inch retina display, you can add
GRUB_GFXMODE=2880x1800
to the /etc/default/grub file, than run sudo update-grub, and reboot.
Next you have to find lines in /usr/share/glib-2.0/schemas/org.gnome.desktop.interface.gschema.xml file
<key name="scaling-factor" type="u">
<default>1</default>
and change the scaling factor
<key name="scaling-factor" type="u">
<default>2</default>
than run sudo glib-compile-schemas /usr/share/glib-2.0/schemas, and reboot.
It worked on my MacBook Pro 15,4 (2017) and your Ubuntu will look more "retina" Good luck.