@Brecht Van Lommel (brecht) - Other code calling BLF_size does this:

I don't mind doing it that way, but it adds to the confusion, all caused by our treatment of U.pixelsize. Although that is a float it is a whole number line width. So if the UI scale is 1.6X then it is 1.0f, but at 1.8X that is 2.0f. Keep the scale the same but turn on "wide lines" and it might become "3.0f"

BLF_size(fontid, fstyle_points * U.pixelsize, U.dpi);

So it is easy to look at the above and wonder how this works when pixelsize is a whole number, and you'd assume DPI was something like 72. It only works because U.dpi is really actual DPI divided by that pixelsize. So the multiplication by U.pixelsize here is not a scaling at all but a correction for that dumb division. We really need to fix that.

This is probably the most explanatory way for it:

diff --git a/source/blender/editors/interface/interface_panel.c b/source/blender/editors/interface/interface_panel.c
index c7f2eb230cb..aacba3d7756 100644
--- a/source/blender/editors/interface/interface_panel.c
+++ b/source/blender/editors/interface/interface_panel.c
@@ -1357,9 +1357,8 @@ void UI_panel_category_draw_all(ARegion *region, const char *category_id_active)
 
   BLF_enable(fontid, BLF_ROTATION);
   BLF_rotation(fontid, M_PI_2);
-  // UI_fontstyle_set(&style->widget);
-  ui_fontscale(&fstyle_points, aspect / (U.pixelsize * 1.1f));
-  BLF_size(fontid, fstyle_points, U.dpi);
+  int dpi = U.dpi * U.pixelsize;
+  BLF_size(fontid, fstyle_points / aspect, dpi);
 
   /* Check the region type supports categories to avoid an assert
    * for showing 3D view panels in the properties space. */

The point is not that I think the existing convention is good, in fact I believe we should remove usage of U.pixelsize and U.dpi from the code entirely.

But it's better to be consistent so that when we do refactor, we don't have to wonder why this one case was doing something different, even if in isolation it might be more clear.