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.

• The 60D chassis is polycarbonate resin (plastic) with glass fiber on an aluminum chassis and moderately weather sealed. The 600D chassis is polycarbonate resin with glass fiber. It uses some internal stainless steel, but it is not really a metal chassis.
• The 60D uses the LP-E6 battery (2.9 oz./82g), the 600D uses the LP-E8 (0.8 oz./23g), so 60 grams of the total difference is the battery weight. That is a little more than 1/4.
• The volume of the L x W x H measurements of the 60D is about 13% larger than the 600 D. When sitting them side by side, though, the boxy, rectangular 60D appears more than 1/8 larger than the angular 600D. Assuming uniform density, 13% of the 560g mass of the 600D is 72 grams., that is roughly another 1/3 of the difference.
• The 600D viewfinder uses a pentamirror which is basically a hollow construct with five mirrors for walls. The bottom and rear are 'two-way' mirrors to allow light in from the bottom and out through the back. The 60D uses a pentaprism, which is a similarly shaped but solid piece of fairly dense material.