This is a part of the effort to make all panel icons have consistent margins, and improves the display of system tray icons in prticular now that they fill the available space by default.
Before, 46px panel (default):
After, 46px panel (default):
Before, 36px panel:
After, 36px panel:
cc @teams/vdg @mart
Looks to me like we might want to also adjust the Task manager background effects a bit too.