Three-dimensional images may seem to be in proper proportion when they actually are not.
First, consider a (1-dimensional) line segment.
Finally, consider a (3-dimensional) cube.
Suppose a company decides to illustrate that its profits have doubled by showing a small money bag (representing their previous profit) next to a double scaled money bag (representing their recent profit). While it may seem okay because the bigger bag is twice as tall, we must keep in mind that it is also twice as wide and twice as deep.
The depth is only a perception, because the images would be printed on a paper surface. Note that even though the images are shown on a 2-dimensional paper surface, as with a photograph, the objects pictured are of course 3-dimensional.