There is a subset of icons in each folder that are smaller 
than the size listed on the folder name. These icons are
smaller to match the smaller toolbar buttons for each of 
these specific icons. 

Those icons are:

first.png
last.png
next.png
previous.png
rotccw.png
rotcw.png
zoomin.png
zoomout.png
zoomsel.png

These 9 icons are size:

12x12 in the small (16x16) folder
16x16 in the medium (24x24) folder
24x24 in the large (32x32) folder
32x32 in the xlarge (48x48) folder