The exchange traded fund industry has quickly accumulated over $2 trillion in assets under management, with investors piling into cheap broad market, smart-beta and some sector fund options.

The Vanguard 500 Index (NYSEArca: VOO) was the fastest-growing ETF over the three years ended December 31, growing to $27.6 billion in 2014 from $2.4 billion in 2011, reports Charles Stein for Bloomberg.

The Bloomberg ranking considers one-, two-, and three-year asset growth where one-year growth is weighted at 50% and two- and three-year growth are weighted 30% and 20%, respectively.

The rapid growth in VOO may be due to its cheap expense ratio of 0.05% but an average 1.4 million daily volume, which suggests that investors are utilizing VOO as a buy-and-hold option. In contrast, the SPDR S&P 500 ETF (NYSEArca: SPY), the largest S&P 500-related ETF, has a 0.09% and an average volume of 111.2 million shares, which indicates that many large players are using SPY to go in and out of the market.

Other dirt-cheap ETFS also made it on to the fastest-growth ETF list, including the Schwab U.S. Large-Cap ETF (NYSEArca: SCHX) and Schwab U.S. Broad Market ETF (NYSEArca: SCHB), the two cheapest ETFs on the market with a 0.04% expense ratio. Additionally, the Schwab ETFs trade commission-free on the Charles Schwab OneSource Platform. [Build a Dirt-Cheap Portfolio With These ETFs]