I also had this issue, until a few minutes ago, on Ubuntu Gnome 17.04, on a 3840x2160 Dell laptop display.
$ gsettings get org.gnome.desktop.interface scaling-factor
$ gsettings get org.gnome.desktop.interface text-scaling-factor
Then I opened the Gnome GUI app "Tweak Tool" and changed the Windows -> HiDPI -> Window Scaling there. I changed it from 1 to 2, then back to 1. Then I quit Insync, and restarted it. Now its UI looks correct.
So touching the window scaling setting using the Tweak Tool seems to be a workaround. Hopefully the issue can be fixed better some other way...
Please let me know @lpugoy if I can assist with any further testing or data.