Market capitalisation is the price of one share multiplied by the number of shares. It is the market's answer to "what is this whole company worth?" — and it decides how much space a company takes in an index fund.

The size buckets

The boundaries are conventions rather than laws, and they shift over time, but the usual framing is:

  • Large cap — the household names. Established, widely covered by analysts, generally less volatile, often dividend-paying.
  • Mid cap — past the fragile stage, still with room to expand. Historically an interesting middle ground between risk and growth.
  • Small cap — smaller businesses, thinner trading, more volatile, more likely to fail and more likely to multiply.

Market cap is not price

This trips up beginners constantly. A share costing 5 is not "cheaper" than one costing 500 — it just means there are more of them. A company with a billion shares at 5 is worth more than one with a million shares at 500. Share price alone tells you nothing about size or value.

Why it decides your index fund

Most index funds are market-cap-weighted, meaning each company's slice matches its share of total market value. Two consequences follow, and both are worth understanding:

  1. You automatically own more of the biggest companies. A broad "whole market" fund can still be dominated by its largest few holdings.
  2. The fund rebalances itself. As a company grows, its weight rises without anyone trading — which is a large part of why index funds are so cheap to run.

The small-cap trade-off

Smaller companies have historically delivered higher long-run returns, which is not a free lunch — it is compensation for genuinely higher risk, deeper drawdowns and long stretches of underperformance. Anyone tilting toward small caps needs the time horizon and the stomach to sit through those stretches.

Free float matters too: indices usually count only shares actually available to trade, so a company where founders or a state hold most of the stock has a smaller index weight than its headline market cap suggests.

Summary

Market cap tells you a company's size, its likely behaviour, and its weight in your fund. It tells you nothing about whether it is a good investment — that is what valuation is for.