Why is the camera in the viewport smaller than the focal length suggests it should be? I launched Blender, in the default "Startup file". I changed the Camera Location X, Y and Z, halving them.

Then I changed the "Units" in the "Scene" to Millimeters as you can see in the following screenshot; in the same screenshot you can see the cube "Dimensions" is 2mm.



The rendered image is fine:



But I do not understand the mutual size of the cube and the camera Focal Length: the cube side length is 2mm and the focal length is 35mm, so the focal length should be ~17x longer than the cube side but in the following screenshot the focal length seems shorter than the cube side.



What am I missing?
The size of the camera in the viewport is not the same as the focal length. Scaling the camera object won't change the focal length or the rendered image, but will change the size in viewport.

For focal length = 32mm, the camera object will look like the principal plane is 1 BlenderUnit away from the camera center. This is probably an intuitive scale so that the camera object does have a fitting size for a usual scene. If it were acutally 35 BlenderUnits big, everyone would scale it down right away.