Below you can see the front-view size comparison of the Nikon D3500 and Olympus E-M10 II. The Olympus E-M10 II is clearly the smaller of the two cameras. Its body is 4 mm narrower, 14 mm shorter and 23 mm thinner than the Nikon D3500.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.