Brazil's central government started the year with a smaller-than-expected primary surplus as spending outpaced revenue growth, Treasury data showed on Wednesday. The government posted a primary surplus of 86.9 billion reais ($16.87 billion) in January, below the 88.8 billion reais forecast in a Reuters poll. Expenditures rose 2.9% from a year earlier, while net revenue increased 1.2%, highlighting leftist President Luiz Inacio Lula da Silva's challenge in balancing public accounts largely through revenue-raising measures.