In terms of the bar bar height, from what I can tell we are using the same vertical space as the old default theme did. Could someone extrapolate on the aversion to the current height? Is the issue that there is now more contrast in the tab bar?
This is probably also a good time to point out that you can customize a theme by creating file in your
Packages/User/ folder with the same filename as the theme. For the Default theme, this would be
Packages/User/Default.sublime-theme. In this file, you can add custom theme rules. You can learn about the rules at https://www.sublimetext.com/docs/3/themes.html.
Here is an example of changing the default row spacing of the sidebar:
"row_padding": [16, 1, 16, 1],
In the case of the tabs, you'd have to create your own textures that are shorter than the default tabs to make it shorter.