Investors are piling in to exchange-traded products, betting that stock-market volatility will increase from rock-bottom levels. But while they wait to cash in on a big spike, their returns are dwindling due to a quirk of the market. The largest product that follows the performance of Cboe Volatility Index futures, the Barclays iPath S&P 500 VIX Short-Term Futures ETN, has increased assets by more than 300% this year to over $1 billion. The attraction is that if the record stock rally withers, market vol ...