Woah!! I just came to this thread to suggest the exact same thing. It would be great to have an option to adjust the density and size of the items.
The font size and spacing is high by default on Horizon compared to the other themes. I understand its done to give it a more modern look, but power users may prefer to see more dense layouts. Put simply maybe 90% zoom but elements adjusted to fit the negative space left out.
For us designers and developers this is not immediately obvious as we work on high resolution screens, but for users who are still on lower resolution screens, everything looks too large.