In Mathematica 13 the font and everything else in notebooks and the size of toolbars is too big, compared to Mathematica 12.3 (with all the settings the same, magnification set at 100%).
It seems, the "global magnification" setting actually works (for the purpose of correcting everything being big), but does not affect the palettes (but per-palette magnification setting is remembered, so not a big problem), does not affect the about and welcome dialogs, and does not affect some elements in the notebook windows (the "+" at the top of the notebook and the cell braces at the right). Here is Mathematica 13 with global magnification 75%:
Here is Mathematica 12.3 with global magnification 100%, the font size is the same, but the "+" and braces are smaller:
The welcome screen, about dialog, etc, still remain too big, because global magnification does not affect them.