If you just look in terms of aggregate market cap, defense companies are some of the larger companies in the world - i.e., Lockheed Martin is valued over $100B. That's a good sign that it's possible to build a big company in the space, which is all you need.
Lockheed Martin used to be Lockheed and Martin, MDD used to be McDonnel and Douglas, Northrop Gruman, well, you get it. Those companies were built, the were merged. And they are the first on the big defence budgets, and honestly also the only ones being able to deliver modern defence and weapon systems (regardless of delays and cost over runs).
Lockheed Martin was formed by the merger of Lockheed and Martin Marietta and Martin Marietta was formed by the merger of Martin and American-Marietta, and American-Marietta was formed by the merger of American Asphalt Paint Company and Marietta Paint and Color Company.