You always want more run area than coop. The straight sides would reduce the size by about 1/4. Your chickens will spend most of their waking hours downstairs.
The run part is approximately 8x6 or 48 sq feet. Not that much.
They are slanted in the bottom because it's all one piece of wood. If you did the top in an A and the bottom straight, you would have to cut all the piece separately and than how are you going to join them? The A construction is as easy as it goes. It is also more stable and less apt to be...