I played around a bit with the GTK themes and this can be fixed by changing putting something like this in your gtkrc (for the respective theme):
gtk-icon-sizes = "gtk-large-toolbar=32,32:panel-menu=24,24:panel=24,24:gtk-dialog=64,64"
and you might also have to delete some "unwanted" icons in your icons folder which do not have the right size 32x32.
I played around a bit with the GTK themes and this can be fixed by changing putting something like this in your gtkrc (for the respective theme):
gtk-icon-sizes = "gtk-large- toolbar= 32,32:panel- menu=24, 24:panel= 24,24:gtk- dialog= 64,64"
and you might also have to delete some "unwanted" icons in your icons folder which do not have the right size 32x32.