Below you can see the front view size comparison of Canon M3 and Canon M10. Canon M10 is clearly the smaller of the two cameras. Its body is 3mm narrower, 1mm shorter and 9mm thinner than Canon M3.
Canon M3 vs Canon M10 - Front View
Canon M3 vs Canon M10 - 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