Basically, the investors had built their accounts up to over 500,000 BTC collectively via compound interest, so they feel like they lost more bitcoins than they actually had. Those bitcoins never existed. Only the principle deposited and whatever pirate made from his market shenanigans were available to the fund.
Okay, I understand what you are saying. Before I doze off (it's late here!), I will publish my thoughts on a ballpark guess (not an estimate, just wild speculation).
Let T be 500000
BTC (the total amount in the fund), I be the total interest credited. As the scheme was not 500000
BTC from the start, there must have been a growth. Assume that growth started April at 10000
BTC. This works out to 25% growth in the scheme per week, which is reasonable. The geometric sequence based on this growth extrapolates to 140000
BTC interest, which we can round up to 150000
BTC. This is reasonable, and implies the total amount stolen is near 350000
BTC.