How did French kings increase their power in Middle Ages?

"Expand their power" is a meaningless concept. A king must delegate his power to generals and bureaucrats. The more he expands his borders, the more generals and bureaucrats he needs to take care of details. In China, the bureaucrats held so much power that on one occasion they felt some threat to their jobs, so they scuttled the entire navy to protect themselves. That is why China has had so little influence in the age of exploration.

Your typical power freak would think this answer is totally bonkers.