When One Position Gets Too Big
What to do when a single holding grows into a large share of your portfolio: the case for trimming, the case for letting it run, and practical middle paths that account for taxes.
The short version
A successful investment can grow until it dominates a portfolio, turning a modest bet into a concentrated one without any decision to concentrate. The risk is that one company's bad news now does serious damage. The opposite risk is selling a rare long-term winner too early. Common middle paths are trimming back to a size limit, stopping new purchases, directing new money elsewhere, or trimming gradually to spread out taxes. The right answer depends on your limits, the investment's value, and what a fall would do to your goals.
By the end of this lesson, you'll be able to
- Calculate how a winning position's share of a portfolio grows
- Weigh the risk of concentration against the cost of selling a long-term winner
- Describe practical ways to reduce a large position, including tax considerations
Why this matters
This is a good problem to have, and it's still a problem. A stock that quadruples can go from a small slice of a portfolio to one of its largest holdings. At that point, the portfolio's fate depends heavily on one company, which is exactly what diversification and position sizing were meant to prevent.
How it happens
Jordan's $100,000 portfolio held $5,000 of STU Corp, a 5% position. Over several years, STU Corp's shares quadruple, while the rest of the portfolio grows from $95,000 to $105,000.
| Then | Now | |
|---|---|---|
| STU Corp | $5,000 | $20,000 |
| Everything else | $95,000 | $105,000 |
| STU Corp's share of the portfolio | 5% | $20,000 ÷ $125,000 = 16% |
| Portfolio loss if STU Corp halves | 2.5% | 8% |
Jordan's rule caps any single stock at 10%. STU Corp is now well past it, purely through success. STU Corp is fictional and the figures are hypothetical.
Two risks, pulling in opposite directions
The case for trimming. A big position means one company's bad news, a lost customer, a failed product, an accounting problem, now hits the whole portfolio hard. If your sizing rule exists for good reasons, those reasons don't disappear because a position grew on its own. And a large gain often means a higher price relative to value, so it's worth asking whether the stock is still worth more than it costs.
The case for letting it run. Research covering US stocks since 1926 found that a small share of companies, the best 4%, accounted for the market's entire net gain.1 Long-run winners are rare and valuable. Selling one too early, especially when the business is still strong, can mean giving up most of the gain still to come.
Both are real. The answer usually lies between them, guided by the question from earlier in this part: would you buy this position today, at this price and this size?
The rest of this lesson is for members
Still to come in this lesson: Practical middle paths, Employer stock: a special case, the key takeaways, and 2 quiz questions with explanations.
Membership opens every lesson in the curriculum, the capstone projects, the interactive tools, and the members-only channels in the Hyperion Entities Discord. The first lesson of every module stays free.
Go deeper
- Position SizingPart 5: Building a Portfolio
- Concentration vs. DiversificationPart 5: Building a Portfolio
- Tax Considerations When SellingPart 6: Strategy, Behavior, Accounts, and Taxes
Notes and sources
- Hendrik Bessembinder, Journal of Financial Economics 129(3), 440-457. Do Stocks Outperform Treasury Bills? (2018). Checked 2026-10-05. Back to text
- Internal Revenue Service. Topic no. 409, Capital gains and losses. Checked 2026-09-30.