Politicians often use the phrase "belt-tightening" as a metaphor for fiscal austerity. This refers to the act of tightening one's belt rather than buying a new one if one has lost weight from (presumably) hunger in difficult economic times.
In this metaphor, food would be the equivalent of tax receipts; the emaciated government has not been able to consume enough as it has in the past because of an economic recession or depression. Presumably, then, because of this lowered consumption, the government has lost "weight."
If taxes are the way that the government consumes, then exercise should be the way that the government spends--the expenditure of energy via food or money. The argument for austerity is that, with less food, the government should work less; it should do less of what it had done before because it no longer has the same energy level.
Purchasing or not purchasing clothing, however, just doesn't fit in this extended metaphor. Clothing is the ornamentation or protection of the body; it is not directly connected to the process of eating and exercising, the metaphorical activity of the "body politic." Moreover, not buying new pants doesn't work as a metaphor for budget slashing. It is anti-stimulus, anti-Keynesian for sure, but it is not pro-budget slashing, but rather pro-stasis. It makes a symbolic argument for not starting any new innovative projects during the "difficult economic times," not stopping what you have. The "belt-tightening" metaphor does not say that you have to sell off your pants because you are poor and they no longer fit you.
The "belt-tightening" language conflates two metaphor: (1) government as body (with the adversity of hunger) and (2) government as family home (with the adversity of poverty).
So, let's look at "belt-tightening" again. It says (1) You are poor. (2) You are hungry because you are poor. (3) Because you are poor and (ergo) hungry, your clothing is too big. (4) Because you are poor, you cannot afford new pants for your hunger-induced smaller waistline. (5) As a solution, you tighten your belt and do not buy new pants. (6) You, however, remain hungry and poor, and the lack of new pants does not make you less hungry nor less poor than you were before. (7) Your life still sucks, and you have done nothing to improve it.