Do the following:
1. Use Google/ET to find out names of the major futures exchanges worldwide.
2. For each exchange, go to their website and look up the contract specs and daily volume/open interest for each contract that is listed.
3. Use this information to calculate the $ (Euro, Yen, whatever) value of the volume/open interest for each contract (Notional*V or OI*Exchange Rate).
4. Make a chart showing this number for each contract across all exchanges. This will give you your answer.
5. Post your results here so that other may learn.