If that unlucky player happened to have many large ships in the destroyed system, it's a major victory. The same goes for revolts. A system with several green pieces can be quickly taken over. By diversifying your own ships, avoid creating a potential concentration in your own systems. When possible, sacrifice large ships to have the most growth, the most diversity, the fastest movement, the strongest kill.
Hoarding is important, especially in a long game.
After you have built up a number of ships in several systems, try to control all the large ships in one technology. Expand to take more systems, even if it requires creating more systems, so that you can hoard more technology. Hoarding large red ships is particularly crucial.