I had the same problem, found the tutorial here:http://forums.linuxmint.com/viewtopic.php?f=208&t=117665&p=651427&hilit=panel+icons#p651427
To solve this, just go to Cinnamon Settings (right click the panel) >> Panel >> check the two boxes:
"Use customized panel size (otherwise it's defined by the theme)"
"Allow Cinnamon to scale panel text and icons according to the panel heights"
This solved my problem without restarting Cinnamon, though the tutorial said i have to restart cinnamon.