Why Can a Market-Cap-Weighted Index Become Concentrated in a Few Companies?
A fund can hold hundreds of companies and still end up dominated by a handful of them, not because anyone chose it that way, but because of how cap weighting works.
The short answer
A market-capitalization-weighted index gives each company a weight based on its total market value relative to the whole index, which means that as some companies grow much larger than others, their weight in the index grows right along with them. Over time, especially during periods when a small group of companies rises sharply in value, this can leave a supposedly broad index dominated by a handful of names, even though it still technically holds hundreds of companies.
The mechanics of weight drift
In a cap-weighted index, no one manually decides that a company deserves a bigger slice — the market does that automatically through price changes. If a handful of companies’ share prices rise much faster than the rest of the index for an extended stretch, hypothetically, their combined share of the index’s total value can climb from a modest starting point to a much larger one, all without a single rule change or manual intervention. The index is simply reflecting the market’s own valuation, which is the entire point of cap weighting, but that also means it has no built-in limit on how concentrated it can become.
Why this happens more in some periods than others
- Uneven growth across companies. When gains are broadly shared across many companies, cap weighting stays relatively balanced. When gains cluster in a small number of companies, weighting concentrates there too.
- No rebalancing to offset it. Unlike an equal-weight index, which resets holdings back to an even split on a schedule, a cap-weighted index has no mechanism pulling concentration back down — it simply reflects whatever the market produces.
- Sector effects compound it. If the companies growing fastest also happen to share a sector classification, concentration in the index can show up as both company-level and sector-level concentration at the same time.
What this means for diversification
An index fund is often chosen specifically for diversification — broad exposure across many companies rather than a bet on a few. Concentration drift doesn’t necessarily undo that diversification entirely, since the fund may still hold hundreds of companies, but it does mean the fund’s actual behavior can end up driven disproportionately by a small handful of holdings, which is a different risk profile than the “broad market” label might suggest. This is part of why concentration figures are worth checking periodically rather than only at the time of a first purchase.
How to check for concentration in practice
Checking a fund’s top holdings and what share of the total portfolio they represent is a simple way to see how concentrated a cap-weighted index has become at any given time, rather than assuming a large number of total holdings automatically means balanced exposure. This is one of the tradeoffs of cap weighting versus alternatives like equal weighting: cap weighting requires no ongoing rebalancing and keeps costs low, but it also has no built-in check against concentration.
The takeaway
Cap weighting isn’t a flaw in index design — it’s a deliberate, low-cost way of letting the market set each company’s weight automatically. The tradeoff is that the same mechanism offering low turnover and low cost also allows concentration to build quietly over time, which is worth checking for rather than assuming away just because a fund’s holdings count looks broad on paper.