Okay Jack, here is my understanding of what you are asking. Everyone else, feel free to correct me on the concept, but lets try not to muddy the water on the tight details.
What I am going for here is the broad idea.
First, Lets say that you have an 8X10 blank page. There are NO pixels on this page so you have the classic white cat in a blizzard picture.
Then you decide to put on this page 14.2 megapixels of an Iris. This picture has bricks in the background, all in all a very pleasing picture to look at.
But it needs to be made more pleasing. So lets take out most of the stalk so the Iris almost completely fills the picture. You crop the image.
Cropping, in addition to taking out parts that you don't like, also removes PIXELS! So, instead of a 14.2 megapixel image, you now have a 4.5 megapixel picture.
Since fewer pixels are filling the picture, but the image size is the same 8X10, then the pixels HAVE to be larger. Not so large that the image begins to pixelate, but it looks as if
the image is closer to you. Make sense?
Now, if you put these same 4.5 megapixels on a 13X19 sheet, THEN each pixel would be so large that the image would begin to look very blocky and stepped. This is the dreaded pixelation. I know that I have dumbed this down and that you probably already knew all of this. I am willing to bet that you have just never looked at it this way.
Hope this helps.
Pete