For a while, the SPDR S&P 500 ETF (SPY) was the only exchange-traded fund (ETF) with more than $100 billion in assets under management. That changed late in the first quarter of 2017 when the iShares Core S&P 500 ETF (IVV) joined the sparsely populated $100 billion club.

Though still sparsely populated, the $100 billion ETF club is growing. There are now three funds in that group after the recent entry by the Vanguard Total Stock Market Fund (VTI). As of the end of July, VTI had $101.8 billion in assets under management, according to issuer data. Year to date, investors have added $4.56 billion in new assets to VTI, a total surpassed by just nine other U.S.-listed ETFs. With month-to-date inflows of almost $167 million, VTI's assets under management tally is fast approaching $102 billion. (See also: Vanguard Total Stock Market ETF.)

Including the ETF and index fund share classes, the Vanguard Total Stock Market Fund is one of the world's largest index funds with $725.8 billion in combined assets. To put that $725.8 billion into context, that means the assets held by the Vanguard Total Stock Market Fund are more than double the market capitalization of Johnson & Johnson (JNJ). Just four members of the S&P 500 have market values north of $725.8 billion.

Investors' affinity for VTI is easy to understand. The ETF provides broad exposure to the U.S. equity market by holding more than 3,600 stocks and does so with a nominal fee. Just a handful of ETFs have lower expense ratios than the 0.04% per year, or $4 on a $10,000 investment, charged by VTI. (For more, see: Is VTI the Best Index Fund?)

IVV, SPY and VTI could soon have company in the $100 billion club. As of July 31, the Vanguard S&P 500 ETF (VOO) had $94.8 billion in assets under management, according to issuer data. Month to date, investors have added another $2.07 billion to VOO.

On a year-to-date basis, VOO has hauled in $8.50 billion in new assets, ranking it third among all U.S.-listed ETFs. IVV, SPY, VOO and VTI are the only ETFs trading in the U.S. with more than $71 billion in assets under management, indicating that it could be a while before a fifth member of the $100 billion club arrives. (See also: A Look At Vanguard's S&P 500 ETF.)