Admin theme Font size is too big

I'll close this as this has been designed with Accessibility in mind. We're following in Claro's footsteps here.
We might introduce some more customization options in the future (like Add ability to change theme density) but for now I'll have to close this as it's out of focus.
Cheers

What you could do is using the provided public://gin-custom.css and add an override for the default font size.
Most elements should scale depending on this. But in terms of both Accessbility & Usability I would definitely advise against it, but that's up to you:

html {
  font-size: 14px; // You're new font-size
}

Since this is an admin theme, there could be only a single user. That single user should be allowed to reduce the font size, since there may be no other users to consider. So I am re-opening, in case this option could be reconsidered ... or it could be added to #3319463: Extend theme density functionality ?

Closing this as works as designed as the font-size has been chosen with a11y in mind.

You can override the default font-size as described (see above) if you have a specific use case.