For the energy sector, Exxon Mobil is the biggest contractor with almost $800 million in petroleum contracts, mostly for the Defense Logistics Agency. That may seem like a lot, but it's a negligible fraction of the energy giant's revenue each year. The same is true for the telecom leader, AT&T, and shipping leader FedEx, which each made a little over $600 million from the government in 2015.

The situation is completely different for the leading firm in information technology, SAIC, which depends on government contracts for 78 percent of its revenue. Private prisons firm GEO Group lead the real estate sector, making about 71 percent of its $1.8 billion in sales from government contracts.

In health care, McKesson Corp. made over $8 billion, most of which came from its lucrative relationship with Veteran's Affairs. That's about 4 percent of the pharmaceutical distributor's revenue.

For companies classified as financial firms, Berkshire Hathaway was the largest government contractor. But the company didn't earn that position through its financial businesses — most of its government revenue came from its nonfinancial holdings, including providing metals to the U.S. Mint, mobile homes for FEMA and flight training for the FAA, according to government data.

Not counting state or local spending, the federal government paid almost $240 billion to its top 100 contractors in 2015. Those are big sums, and those companies are going to be noticing everything Trump says.