Hello. The 77mm heatsink is likely for 115 W and 130 W processors whereas 55mm heatsink is for processors of power output under 115 W.
Hi All,  I have noticed that the dell M620 have two versions of heatsinks. One is 55mm and the other is 77mm , I have both at hand, can someone explain what is the difference in them?  
Dell M620 heatsink