To add on, please DO NOT confuse this with lens distortion.
I had huge troubles trying to explain this to one of the CS "portrait masters" who insisted that his lens was distortion free based on lens tests on the net.
You have to think of it in extreme terms. Just look at the standard UWA landscape, where a huge key element is emphasized, say, a rock with background as sea, water and sky. Is the rock larger than the cloud in the distance? Definitely not, but the focal length makes this so.
Now back track about 200 steps with the same focal length (UWA). Look at the rock and the sea now, does the rock seem more in proportion to what your eyes see? For sure.
Now replace the rock with someone's nose, for example, and picture it as the foreground with the ears and the eyes as the background. The use of a too-wide focal length when you are too close can over-emphasize the features nearer to the lens surface, making them larger than is proportionally so. This can be used to great effect, but most of the time it is frowned upon. This applies to all focal lengths - for example if you shoot a head shot with a 35mm lens, i.e. fill the whole frame with the head, you would get it too. There is no need to be over pedantic about this, but knowing it's there is important, so that you can determine whether you WANT it there.
Hope this helps.