Why Market Cap Matters for Diversification

When most people talk about the market, they are usually referring to the S&P 500. This index has become the standard benchmark for U.S. equities. But the way the S&P 500 is constructed makes it important to understand not just which companies are in it, but how much weight each one carries.

The index is weighted by market capitalization, which is the total value of a company’s outstanding stock. That means the larger the company, the greater its influence on the overall performance of the S&P 500.

Over the past few years, a small group of companies has come to dominate the market. Firms like Apple, Microsoft, Nvidia, Google, and Meta have grown so large that the top 10 companies in the S&P 500 now represent about 35 percent of the index.

Out of roughly 500 companies, 10 account for nearly half of the returns. This creates a very top-heavy market dynamic.

There are some parallels between today’s market and the late 1990s. Back then, internet and technology stocks drove market gains, and optimism reached extreme levels. Today, technology again dominates performance, but the environment is not identical.

• Investor sentiment is more cautious now than it was in 1999, which tempers bubble-like behavior
• Valuations of the largest companies are stretched compared to smaller peers, creating a significant gap
• This concentration means investor portfolios may be less diversified than they appear if they only track the S&P 500

While large companies have outperformed for more than a decade, history shows that leadership shifts over time. In certain periods, such as the 1970s, early 2000s, and after the financial crisis, smaller companies significantly outperformed their larger counterparts.

Today, small and mid-cap stocks have

• Much lower valuations relative to large-cap companies
• Poor investor sentiment, which often sets the stage for stronger future performance
• Potential tailwinds if interest rates ease, lowering borrowing costs for growing businesses

This does not mean small caps will lead immediately, but it suggests investors should consider whether their portfolios are too heavily tilted toward mega-cap stocks.

Owning dozens of companies is not the same as being diversified if they all behave similarly. True diversification includes investment across industries and across sizes, including large, medium, and small.

Smaller companies may not be market leaders today, but historically, these shifts happen in cycles. Being positioned before the next change in leadership can be a valuable long-term strategy.

The dominance of mega-cap stocks has created both opportunity and risk. While the largest companies will continue to play an important role in markets, investors should not overlook the potential benefits of small and mid-cap companies, especially given today’s valuation gap.

Consider reviewing your portfolio to ensure you have exposure across different company sizes. Contact Bradley, Foster & Sargent today to discuss your investment strategy.