Macaroni penguin, Cooper Bay, South Georgia. I think his name was Christian, but there are around 2 million of them, so I might be confusing him with his brother. Canon EOS-1D X and 200-400mm 1.4x lens.
Why do photographs made with the same size sensor look so different? For instance, your smart phone or compact camera might have a 20-megapixel sensor, but the photos it produces don't look nearly as good as those taken with a 20-megapixel DSLR sensor.
Yet when you are printing, the literature talks about needing so many pixels per inch or centimetre to make a 'good print'. However, what's really being discussed here is 'aliasing' or the jaggies!
This is a non-scientific explanation.
For digital photography, we want lots of pixels so when we make a print or display an image, we don't see jagged or 'stepped' diagonal lines (the errors are called 'aliasing').Think of a chessboard with its squares and imagine what would happen if you tried to use these squares to draw a diagonal line. You'd see all the steps. Now imagine that same board with one million squares. When you draw a diagonal line, the steps are still there, but they are so small you can't see them. Following this logic, a 20-megapixel image from any camera will hide the 'jaggies' pretty well.
However, there's more to making a quality image than just eliminating the jaggies. You need high quality pixels as well and the answer lies in the physical size of the sensor, not just the number of pixel sites on the sensor.
Larger sensors can hold larger pixel sites, and larger pixel sites generally mean better image quality. A 20-megapixel, full-frame DSLR sensor physically measures 36x24mm and each sensor or pixel site might be, say, 6 microns in size. In comparison, a 20-megapixel sensor on a compact camera or a smartphone is much, much smaller and each pixel site might measure, say 2 microns.
This is one of the reasons there is a quality difference. A 2 micron site can't hold as many photons of light as a 6 micron site, so there is less information to work with when it comes to mathematically turning the photons into pixel information. Also, with such small sites, it's easier for the light to 'overflow' from one site to another, contaminating image quality. So, while a compact camera or a smart phone might have as many pixels as a DSLR, the quality of those pixels is not as good (with today's technology). They can look great on a small LCD screen, but as the reproduction size increases, the lack of quality becomes more apparent.
There are a lot of other factors that go into the final image quality as well, one of the main ones being lens quality, but I'll save that discussion for another day.