The only trick is that Thorium UI is rendering at 2x. It doesn't seem to follow global env variables for GDK_SCALE or Wayland scaling. Assume you're running on X11?
Discussion
example i3 keybinding to fix this: (adding GDK_SCALE=1)
bindsym $mod+b exec --no-startup-id GDK_SCALE=1 thorium-browser
example .desktop file to fix this: (adding GDK_SCALE=1)