Below you can see the front view size comparison of Canon 6D and Canon 80D. Canon 80D is 6mm narrower and 6mm shorter than Canon 6D but it is also 8mm thicker.
Canon 6D vs Canon 80D - Front View
Canon 6D vs Canon 80D - Top View
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.
Camera Size Comparison
Select Two Cameras for Size Comparison