Cameras and the human eye both have a characteristic called depth of field. The closer something is to your eye then the smaller the distance range over which you will have things in focus. You can see this if you hold one thumb out at arm's reach and the other close to your eye. Notice the background; it will be slightly blurry. As you move the far thumb toward your eye, notice how things in the background get more and more blurry.
The near-field and far-field blur giving a shallow depth of field image is one of the cues your brain uses to determine distance. Your brain also knows that distance*area covered is proportional to size. So if you lie to your brain by making "far" and "near" items more blurry, your brain wants them to be very close, meaning that they must therefore be very small.