A pixel is a point in space rather then a little square. An image is then reconstructed from point samples, using different reconstruction filters. On of the poorest of these is the box filter. This on just applies a box to a point sample.

“When one “magnifies” or “zooms in on” an image in most popular applications, each pixel appears to be a little square. The higher the magnification or the closer in the zoom, the bigger the little squares get. Since I am apparently magnifying the pixel, it must be a little square, right? No, this is a false perception. What is happening when you zoom in is this: Each point sample is being replicated MxM times, for magnification factor M.” (Smith, p. 7)