Why does the Government not disclose the overall size of our reserves?

It is not in our national interest to publish the full size of our reserves. Our reserves are a strategic asset, and especially so for a small country with no natural resources. It is a key defence for Singapore in times of crisis. This could include a macroeconomic shock of unprecedented scale that causes an outflow of capital in excess of what MAS holds. It could also be emergency scenarios precipitated by state or non-state actors that threaten our economy and livelihoods, or even Singapore’s existence as a nation.
MAS and Temasek publish the size of the funds they manage. As of 31 March 2024, the Official Foreign Reserves managed by MAS was S$498 billion and the size of Temasek's net portfolio value was S$389 billion.
It is the size of the Government's funds managed by GIC that is not published. What has been revealed is that GIC manages well over US$100 billion. Revealing the exact size of assets that GIC manages will, taken together with the published assets of MAS and Temasek, amount to publishing the full size of Singapore's financial reserves.
Just as our defence forces do not reveal the full extent of our weaponry and military capabilities, it would be unwise to reveal the full and exact resources at our disposal.