I would say that private property is a source of economic growth because people who have (or are allowed to have) private property want to get more of it.
In a free market economy, the desire for property (and money) is one of the major drivers or economic growth. People will try to create new products and new ways of making products so that they will be able to get more money. This allows the economy to grow as people come up with new things to make and better ways to make things.
So private property gives people a powerful motive to work hard and that often leads to economic growth.
For a more detailed statement of this idea, read the first few paragraphs of the paper that I have linked to below.