The more pixels you shove onto the same physical sensor, the more chance for digital noise to be present during your shots.
Think of it this way: if you draw 12 million squares on a piece of paper and then draw 15 million squares on the same piece of paper, in which squares do you think you'll be able to see more detail? The larger the square, the more detail that is present. Now think of this like picture. The larger your pixel, the more picture you'll get.
That might not be as clear as I hoped, but I do hope you get the idea
