Some thoughts I have had about the size of the Church savings fund.
1) The whistleblower says the Church currently uses about $6 billion a year in operations (building construction & maintenance, needs of individuals in local areas (rent, food, medical aid, etc.), humanitarian/foreign aid, publications & materials, local budgets, education, etc.). This is paid for by the donations received which are currently around $7 billion a year. It seems logical that when the economy is good donations will increase, and operation costs (fast offering needs, aid, etc.) go down. Therefore, it is not hard to imagine a situation where donations are less than operating expenses.
2) The Church's emergency fund is most likely to be needed in the event of a significant market decline. Given that the fund is invested in the market, it is reasonable to assume that it would also be impacted by the decline. As such, the value of the fund would be significantly less when used than prior to the down swing. As a reference, the DJIA dropped 54% from October 9, 2007 to March 6, 2009.
3) There have been proposals to change the tax status of churches. It is not likely to happen in the next few years, but it is a real risk that could happen in the next 10-15 years. If churches were taxed on donations, then the current donation level would not be sufficient to meet the operating needs of the Church. As such, the earnings in the savings fund would allow the Church to continue current levels of operations.
4) There have also been proposals to remove any tax advantage to making charitable contributions. The reality is, at least in the US, the vast majority of people do not get a tax benefit from charitable contributions (because they are better off taking the standard deduction instead of itemizing). However, the highest value donors would not be able to donate as much should this tax change.
Anyways, my point is that this size of savings seems in line with their stated goals and teachings when I consider some economic scenarios, and the sensitivity of the Church to financial self reliance.