They own a smaller portion of outstanding shares, but the shares they own are worth more. Their value doesn't decrease.
Issuing new shares is dilutive.
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Stock buyback example:
There are 10 shares of ABC Corp, it has a market cap of $100. Each share is worth $10.
ABC corp buys back one share and destroys it, leaving 9 shares.
$100/9 shares=$11.11 per share.
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Dilution example:
There are 10 shares of ABC Corp, it has a market cap of $100. Each share is worth $10.
ABC corp needs $10 so they sell one additional share which the board has authorized. There are now 11 shares.
$100/11 shares=$9.09 per share.
They're not worth more. The number of shares is increased by the amount that was issued. So that larger amount of equity is split among more shares.
A simple example:
Current equity of $100B, 100 shares outstanding. Each share is worth $1B, with 1% of voting power.
Then we issue $10B of equity via 10 shares.
Now we have equity of $110B, with 110 shares outstanding. Each share is still worth $1B, except each share now has 1/110 of voting power, instead of 1/100. Each share now only controls 0.91% of the company. Their ownership has been diluted.
edit: You can throw this comment thread URL into your favorite clanker (Claude, Gemini, whatever) and it will confirm exactly what I'm saying.