The part you're ignoring is future earnings. Assuming a company maintains its earnings, you have a bigger slice of the pie next quarter. If the company trades at the same EPS multiple, your stake definitely has gained value. You're totally ignoring enterprise value.
In a vacuum, nothing has changed about a company's future earnings when they repurchase shares.
The kind of bizarre roundabout way to consider it is if Apple bought 25% of itself with its money mountain (they can't, beside the point), your stake in the company goes up because you own shares in Apple, which in turn owns 25% of itself.
You effectively have more equity in Apple when it buys its own shares.