At the end of 2016, there was $3.546 trillion in combined assets under management for exchange traded funds (ETFs) listed around the world. Some $500 billion of that total resided in smart beta ETFs, one of the ETF industry's fastest-growing segments.

Think $500 billion is a lot of money? It is, but think again in terms of smart beta growth. BlackRock Inc. (BLK​), the parent company of iShares, the world's largest ETF sponsor, said last year it expects smart beta assets to swell to $1 trillion by 2020 before more than doubling to $2.4 trillion by 2025.

At its core, the premise behind smart beta strategies blurs the line between active and passive management. Smart beta ETFs usually feature lower fees than actively managed mutual funds while using weighting methodologies that are more exotic than the capitalization weighting that dominates so many traditional ETFs and index funds.

Fertile ground for issuers of smart beta products have been factor-based ETFs, or those funds focusing on a single or multiple investment factors, such as low volatility, momentum, value and size. With the current bull market in U.S. stocks aging to the point of being one of the oldest on record, it may not be surprising that investors are flocking to value ETFs.

However, investors mulling value ETFs should consider just how much value is left in these funds following last year's massive inflows before jumping in. Here is a look at some popular value ETFs and how devoted investors have been to these funds since last year. (For related reading, see: The Main Attractions of ETF Investing.)

Vanguard Value ETF (VTV)

At the end of last year, VTV, arguably the king of U.S. large-cap value ETFs, had a whopping $53.8 billion in assets under management, according to Vanguard data. On its own, that data point is not concerning, but notable is the fact that roughly 10% of VTV's assets under management arrived into the ETF last year.

VTV follows the CRSP US Large Cap Value Index and bears one of the hallmarks of traditional value ETFs: Significant weights to financial services and energy stocks. With the current bull market aging, few sectors are seen as attractively valued. Relative to long-term averages, energy and financials are perhaps two of a small number of groups legitimately offering value at this point.

Those sectors combine for about 36% of VTV's weight. VTV is favored by advisors and investors for another reason: A low fee. Charging just 0.08% per year, VTV is less expensive than 93% of rival value funds. (For more, see: Top 5 ETFs of the Year.)

iShares S&P 500 Value ETF (IVE)

The iShares S&P 500 Value ETF is another titan among U.S. value funds. This ETF has $13.3 billion in assets under management and as was the case with VTV, a massive chunk of IVE's assets arrived in 2016 as investors poured more than $2.4 billion into the ETF.

Like VTV, IVE has substantial financial services and energy exposure as those sectors combine for over 38% of the iShares ETF's weight. IVE holds 352 stocks and tracks the S&P 500 Value Index.

iShares Russell 1000 Value ETF (IWD)

The iShares Russell 1000 Value ETF tracks the Russell 1000 Value Index, meaning its selection universe is larger than those of IVE and VTV. As a result, IWD holds nearly 700 stocks.

While its lineup is larger than those of its aforementioned rivals, IWD is similar to the other value ETFs. That includes investors' affinity for the ETF as more than 10% of its current assets under management flowed into IWD in 2016.

Financials and energy are IWD's two largest sector allocations, combining for almost 40% of the ETF's roster.

The Bottom Line

Time will tell if smart beta is too hot for its own good, but when it comes to value ETFs, investors are vulnerable to a pullback in energy or financial stocks. If both sectors were to decline simultaneously, the value ETFs mentioned here and others would be highly vulnerable and could easily see massive departures.