Balance in bitcoin-qt suddenly significantly diminished after a small transaction?
The balance that you see is the confirmed balance. This is the sum of the unspent transaction outputs controlled by your wallet that are in transactions that have been confirmed. The confirmed balance can temporarily decrease more than the amount that you actually want to send due to change outputs. However once the transactions confirm, it should be what you expect it to be.
When you make a Bitcoin transaction, it has to spend transaction outputs that were created previously. Because an output can only be spent in full, the transactions that your wallet makes typically create another output called a change output. This is a transaction output that your wallet can spend; it is sending the change back to itself.
When this happens, because the change output is a new output in an unconfirmed transaction, the value in that change output no longer counts towards the confirmed balance. It counts as part of the unconfirmed balance which is shown in the GUI as the Pending Balance. Once the transaction confirms, it will be counted as the confirmed balance.