For decades, the standard advice for index investors has been simple: Buy the whole market, set it on autopilot, and reap the benefits of instant diversification.
The flagship Vanguard Morningstar Total Stock Market Index Fund (available as ETF VTI or mutual fund VTSAX, plus other mutual fund share classes) is often hailed as the gold standard of set-it-and-forget-it investing.
Holding more than 3,500 individual U.S. equities ranging from multi-trillion-dollar tech titans to micro-cap startups, it offers broad exposure across all corners of the U.S. stock market.
However, a quick look under the hood reveals a stark reality: Just seven mega-cap tech companies account for more than 25% of the entire fund’s value.
If a handful of mega-caps control a quarter of a “total market” fund, are investors getting true diversification, or are they taking on hidden concentration risk?
Tech giants’ footprint in broad market index funds hits historic high
Because VTI and VTSAX are market-capitalization-weighted, a company’s influence on the fund is tied directly to its market valuation. As corporate giants have ballooned, their footprint inside broad market index funds has reached historic highs.
The top seven holdings collectively accounted for 29.48% of the fund’s total assets as of Sept. 30, 2026, according to Morningstar. This includes six of the so-called “Magnificent 7” big tech stocks, plus Broadcom.
StockTickerPortfolio Weight (as of 9/30/2026)NvidiaNVDA7.13%AppleAAPL6.63%MicrosoftMSFT5.20%AmazonAMZN3.30%Alphabet Class AGOOG2.76%BroadcomAVGO2.28%MetaMETA2.18%
While approximately 3,500 remaining stocks make up the rest, their collective movements are frequently overshadowed by the heavyweights at the top.
When Big Tech rallies, total market funds surge. But when the tech sector stumbles, even thousands of smaller holdings cannot prevent the broader index from taking a hit.
It’s important that investors understand what their ETFs and mutual funds invest in to help them build a properly diversified portfolio.
Is concentration risk really a threat?
Market concentration isn’t inherently bad. In fact, the top-heavy weightings of mega-cap tech companies have been the primary growth engine powering benchmark index returns over the past several years.
However, heavy concentration creates two main challenges for investors:
1. Sector overlap across your portfolio
If you hold a total stock market fund alongside an S&P 500 ETF, a Nasdaq 100 ETF, or a growth-focused fund, you may have less exposure to a diverse group of companies than you realize. The top holdings across all four funds are virtually identical.
2. The illusion of mid- and small-cap exposure
While VTI technically holds mid- and small-cap stocks, their combined weight is small compared to large caps. Large caps account for approximately 70% of the entire fund’s weight, meaning mid- and small-cap price swings have a minimal impact on overall fund performance.
Armando Arauz via Unsplash; Canva
How Vanguard views the “Mag 7”
Interestingly, Vanguard’s own research suggests that the concentration argument may be slightly oversimplified.
Vanguard analysis notes that while the Magnificent 7 are often lumped together under the “Big Tech” label, their underlying revenue streams are vastly different. For instance:
Microsoft derives revenue from enterprise cloud infrastructure, software, and personal computing.
Amazon generates revenue from e-commerce, cloud computing (AWS), and digital advertising.
Tesla operates primarily in consumer electric vehicles and energy storage.
Meta and Alphabet depend heavily on digital advertising.
Because their business models, customer bases, and risk factors vary, these companies do not always move in unison.
From Vanguard’s perspective, owning these giants still offers operational diversification across the global economy.
3 ways to rebalance your exposure
If you are concerned about market concentration in your core equity holdings, you don’t need to ditch index funds entirely.
Consider these three adjustments to manage your risk:
Incorporate equal-weight index funds: Unlike cap-weighted funds, equal-weight ETFs (like the Invesco S&P 500 Equal Weight ETF, RSP) give every stock in the index the same allocation. This eliminates mega-cap dominance and tilts your portfolio toward value and mid-cap stocks.
Add targeted small-cap and international allocations: To give smaller companies a real voice in your portfolio, consider pairing VTI with a dedicated small-cap ETF (like the Vanguard Small-Cap ETF, VB) or expanding your international equity footprint (via funds like VXUS).
Re-evaluate your total tech exposure: Take stock of all the funds in your workplace 401(k) and taxable brokerage accounts. If you hold sector-specific tech funds alongside total market funds, you may be taking on more concentration risk than intended.
Understand what’s in your portfolio
Vanguard’s Total Stock Market Index Fund remains one of the most efficient, low-cost vehicles for building wealth. However, holding thousands of stocks does not make a fund immune to top-heavy concentration.
Understanding what you own — and how much of it rests on a few mega-cap shoulders — is key to building a resilient portfolio.
Related: Vanguard’s S&P 500 fund outruns its own rising dividend