First off, there's a bug in the Short Term Buffs of the new UI. It displays the 3rd, 5th (etc) with the bigger icon, and the others with smaller icons.
I got to fixing this, but then looked at the OLD UI, and decided to make the same change.
SO....
To make ANY UI display the smaller icons and/or fix the bug, just do the following to EQUI_ShortDurationBuffWindow.xml and also to EQUI_BuffWindow.xml ... Both are in the /uifiles/default or the /uifiles/default_old
You can use any editor... but make sure to save it as a plain old txt file. These changes will be in several places throughout the files.
change from ........ to
<CX>44 .............<CX>34
<CY>44 .............<CY>34
<CX>40 .............<CX>30
<CY>40 .............<CY>30
and VOILA! ... you have smaller versions of the icons. Also the bug in the new UI will be fixed.