This quotation, usually attributed to Thomas Jefferson, is actually by President Gerald R. Ford.
"Let me reaffirm to you today my deep personal conviction that the best way to begin in our efforts is to improve the government we have, not to enlarge it. I do not believe a bigger government is necessarily a better government. May I add this: Please never forget, a government big enough to give us everything we want is a government big enough to take from us everything we have." ~ President Gerald R. Ford's Remarks at the Annual Meeting of the Chamber of Commerce of the United States, April 28, 1975
This quotation is sometimes followed by a variation of an actual Jefferson quotation, "The natural progress of things is for liberty to yeild, and government to gain ground.", Thomas Jefferson to Edward Carrington, Paris, May 27, 1788.
Gerald R. Ford Presidential Library
The Jefferson Encyclopedia