Below you can see the front view size comparison of Nikon D3100 and Nikon D3200. Nikon D3100 is 1mm narrower and 2mm thinner than Nikon D3100 but they have the same height of 96mm.
Nikon D3100 vs Nikon D3200 - Front View
Nikon D3100 vs Nikon D3200 - 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