I think that when government spending is at lower, "normal" levels then an additional dollar of government spending will cause overall private spending to decrease very little, if anything at all. However as government spending increases to a high percentage of GDP (including spending on interest on debt) then an additional dollar of government spending will cause private spending to decrease by similar amounts (or potentially more then a dollar if government spending is high enough).
Apart from the quantum of spending, we should also look at where the money is going in. If it is just being spent to maintain a bloated bureaucracy and fund some pet projects, then it is effectively going down the drain. On the other hand, if it is being channelled into productive investments, it would have a multiplier effect.