"The big problem of small change" is Italian economic historian Carlo Cipolla's way of describing the hoarding of coins when because of inflation the face value of a coin is less than than the value of the metal used to make the coin. This hoarding of coins is one of 3 explanations for the current coin shortage in Buenos Aires.
The other two:
1. Coin-only bus companies were saving coins to sell to businesses at a mark up.
2. The left wing government of Argentina is conspiring to embarrass the right wing government of Buenos Aires in advance of a electronic bus card system that is way behind schedule.