60D = 755g 600D = 560g
Both are built on an aluminium chassis. The only difference I can think of is a pentaprism. Is it really that heavy? Almost + 200g
What are the other heavy parts?
Canon lists the body-only mass of the 60D at 675 grams and the 600D at 515 grams. These numbers are without batteries. Include the batteries and the 60D is the 755g you quote in your question, but the 600D is only 540g.
Also found this http://www.cameradebate.com/2011/canon-60d-vs-600d-t3i/ Some nice comparison