Answer
The tax cut has a smaller effect on GDP than a similarly sized government spending increase since, with a tax cut, households are more likely to save some of that tax cut (than spend all of the tax cut).
Work Step by Step
The opposite (that the tax cut has a larger effect on GDP than a similarly sized government spending increase) could be the case since increased government spending will have to be paid back with future taxes. In turn, when consumers sense a future tax increase, consumers cut spending today. (Thus, the increase in government spending could be nullified by the decreased consumer spending.)
