It was mentioned that two sensors of different dimensions but containing an equal number of pixels (for instance, a 1" x 1" sensor with a million pixels and a 2" x 2" sensor with a million pixels) would give different results if the 1" sensor was expanded (enlarged) to 2".
It should be noted that, if, for the sake of argument, both sensors in their original configuration had pixels butted against each other with no space between them, then when you enlarged the 1" sensor, the pixels would now have space between them in whatever amount was necessary to reach the 2" size.
This would make for a very pixelated, low quality image, much like people would due with dot matrix printers in days of old.
Another way to think of it is, we have all seen the pictures of the head of a pin with the preamble to our Constitution written on it, or a grain of rice with the Lord's prayers written on it.
So, imagine if you took that pin head and stretched the metal until it was the same size as the actual page of the preamble. While technically all the words would still be there, they would be illegible and would take quite a bit of visualization to even "see" the words. That same thing happens, to a much smaller extent, when you expand a cropped sensor version to the size of a full sensor capture. Thus, we now turn the conversation to apples and oranges because we aren't talking about the same thing.
Expand the full frame sensor by the same amount and it also takes a much more magnified look.
The important thing to remember is, the lens captures and transmits the same information irrespective of what is capturing its contents. However, the size of the image (magnification) depends on where you place the focal plane and the quality of the sensor at the focal plane distance.
So, if you took the same setup, removed the back of you camera and let the lens project to a white wall 10' behind the camera, your subject may be 20' tall. Now all you need to do is invent a sensor that can capture a 20' tall image.
So, as the previous person said, an APS-C sensor with 21MP and a full frame sensor with 21MP, you will have larger pixels on the FF and smaller on the cropped, or there will be more space between pixels (less dense) but the captured images will be identical and will only reflect their traits (quality) when changing sizes.