SEP · ISSUE 37 · September 10, 2026

CONCEPT

Why 7 stocks decide if your index rises

You think you buy 500 companies equally. The reality is that a few names rule over the rest.

THE INDEX

500

companies in theory

THE RULERS

decide the move

THE SPLIT

by size

not evenly

THE IDEA

by size

the biggest companies weigh far more than the small ones

Most indices split weight by each company's market value. The giants take up a huge share, so when they move, they drag the whole index even if the other 490 do the opposite.

THE FIGURE

TO GRASP IT

7

giants that move an index of hundreds

It is not a flaw in the index: it is how it is designed. Size rules, and that is why a few names decide for everyone.

In a size-weighted index, a group of just 7 giants can weigh as much as hundreds of companies combined.

GIANT: — A huge company that dominates the index's weight.
WEIGHT: — How much a company influences the index's move.

SIMPLE RULE

KEY IDEA

Diversified is not always what it seems

“Buying the index is not buying 500 equal bets: it is mostly buying the biggest names, and very little of the rest.”

Ronfy Analysis · Editorial

Owning 500 companies does not protect you if 7 of them hold almost all the weight and the movement.

DIVERSIFY: — Spreading your investment so you don't depend on a single bet.
CONCENTRATION: — When few stocks hold most of the weight.

TO SEE IT

EXAMPLE

How much each group adds to the move

Rounded figures to make the point: the first names move more than the hundreds behind them.

Illustrative example: the first names hold most of the index's movement.

CONTRIBUTION: — How much each stock adds to the index's total move.
ILLUSTRATIVE: — A rounded example to explain a concept, not an exact daily figure.

IMPLICATIONS

WHAT IT MEANS

Three consequences of concentration

LESS DIVERSIFIED THAN YOU THINK
Even with hundreds of companies, your result depends mostly on a handful of giants. If they fall, your portfolio falls with them.
THE INDEX CAN MISLEAD
The index can sit at highs while most of its companies fall. You see a green number and miss that inside almost everything is red.
WATCH THE BREADTH
Breadth measures how many stocks rise versus fall. A rally on many legs is healthier than one held up by a few names.

A few names ruling changes how you should read your index and your real risk.

BREADTH: — How many stocks rise versus fall inside an index.
HIGHS: — The highest price level reached over a period.

COMPOSITION

EXAMPLE

How a typical index splits its weight

THE TOP 7Giants that dominate the movement32%

THE NEXT LARGEBig but secondary companies33%

THE LONG TAILHundreds of low-weight companies35%

Seven names weigh almost as much as hundreds of companies combined. That is the heart of concentration.

Illustrative split: the largest names take a huge share against the long tail of the rest.

LONG TAIL: — The large number of small companies that barely move the index.
DOMINATE: — When a small group controls the outcome of the whole.

WATCHLIST

5 EXAMPLE ETFs

Five ETFs to understand concentration

SPY	~765	→ by size	Cap-weighted S&P 500. The giants weigh more than the rest.
RSP	~185	→ equal weight	The same S&P but with every company at the same weight.
QQQ	~590	→ very concentrated	Large tech names: even more dominated by a few.
VOO	~705	→ by size	Another cap-weighted S&P 500; same logic as SPY.
IWM	~230	→ broadly spread	Small companies: the weight is far more evenly split.

These five show the difference between an index ruled by giants and one that is more evenly spread.

ETF: — A listed basket that tracks an index.
EQUAL WEIGHT: — An index where every company weighs the same, regardless of size.

WRAP-UP

Did you know few names move your index?

Understanding concentration helps you know what you actually own. Share it if the idea landed.

One concept a day to invest with your head. Another tomorrow.

Daily briefing · Mon-Fri 16:00 ET

CONCENTRATION: — When few stocks dominate the index's weight and movement.
WEIGHTING: — The share each company holds inside the index.

–