"But this long run is a misleading guide to current affairs. In the long run we are all dead. Economists set themselves too easy, too useless a task, if in tempestuous seasons they can only tell us, that when the storm is long past, the ocean is flat again." - John Maynard Keynes
The GNP is defined as:
GNP = C + I + G + X - M
C = Private Consumption
I = Private Investments
G = Government Spending
X = Exports
M = Imports
Transfer payments, where the government simply takes money from one group and pays it to another, are not included in the definition of government spending for calculation of GNP. However, if the government pays some people to dig a ditch and then pays them to fill it back in, the defined GNP goes up. Really this is just another transfer payment that should not be included because there is no real improvement in the economy. Sadly, much of government spending amounts to hidden transfer payments that don't really increase the size of the economy. This is like a loophole in the definition of GNP that many use to argue for more government spending.
Keynesians predict that if government spending goes up then the GNP will go up. By the formula defining GNP, and because the other variables are slower to change, the immediate impact of a sudden boost in government spending is a boost in GNP. Predicting this increase is too easy and too useless a task. It is nearly "by definition". In this case, it is the short run short run prediction that is trivial and the long run prediction that is interesting, hard, and important.
In the long run, the larger the government is as a percentage of the economy, the slower the economy grows. The government is an overhead that the productive private part of the economy must pay for one way or another. They may pay taxes, or loan it money, or pay an inflation tax. The more economic resources the government controls the less resources are available to grow the real economy.
If you keep applying the Keynesian prescription of more government spending, year after year, the government keeps getting bigger and the real economy suffers. Sadly, today Keynesians are the dominant economists and they have been focusing on the short run far too long.