Below you can see the front view size comparison of Nikon D3100 and Nikon D3000. Nikon D3100 is 2mm narrower and 1mm shorter than Nikon D3000 but it is also 11mm thicker.
Nikon D3100 vs Nikon D3000 - Front View
Nikon D3100 vs Nikon D3000 - 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