NAIROBI (Reuters) - Kenya's gross domestic product (GDP) was assessed to be 25 percent bigger in 2013 after the authorities changed the base calculation year to 2009 from 2001, the government said on Tuesday. Economic output was calculated to be 4.76 trillion shillings ($53.3 billion) after rebasing, up from 3.8 trillion Kenyan shillings ($42.6 billion), the minister for devolution and planning, Anne Waiguru, told a news conference. Growth in 2013 was calculated to have been 5.7 percent after rebasing, up from the previous estimate of 4.7 percent. (1 US dollar = 89.2500 Kenyan shilling)