The political scientist James Payne argued that there is a culture of spending in Congress. Even people elected on a platform of cutting government become enured to higher spending as week after week they hear witnesses saying how much more money is needed and how many more problems could be solved if only you, the great Congressperson, would use your power to spend.
Here is a great illustrative graphic (click to expand) from The New York Times. It shows proposed spending in 1995-96 and 2004-05 from the 30 of the 75 freshmen Republicans elected to the House in the Gingrich revolution of 1994 who remain in the house. Almost all proposed big spending cuts in 1995 but today only 1 (Sue Myrick of NC) continues to propose big cuts. (Yeah Sue!). Almost all of the rest are now big spenders.
It’s somewhat unclear whether Payne’s hypothesis of a culture of spending explains the pattern. It could be that more senior members spend more and thus as the freshmen gain power they increase their spending (thus term limits, for example, would not solve this problem). It could also be a selection effect, perhaps only those who become big spenders are reelected. I tend to favor the latter explanation. We get the government we deserve, unfortunately.