It’s all about the research. We have found that design for smaller screen is all about the priorities. With a desktop screen you put the important things front and center and you often use large images to tell a rich, compelling story. This doesn’t work so well on mobile, where you might have a much smaller viewport and slow download times. You can’t show primary content supported by secondary content in (for example) a right column. You need to get your message across in just a few pixels.