When information-technology providers fight for U.S. government contracts, they’re finding more and more that size matters.

They can either pull back from the business, as military contractors Lockheed Martin and Northrop Grumman did with sales of IT divisions over the past decade, or expand. That’s the response General Dynamics chose with the $9.6 billion takeover of cybersecurity specialist CSRA, a deal that will make the combined company one of the biggest suppliers in the field.