Fitts's Law

Important actions should be big and close to where the user's attention already is. Destructive actions should be small, far from the starting point, and ideally behind a confirmation.

This is why the iOS 'Delete account' button is buried three settings screens deep and shown in all-caps red, while 'Send' and 'Buy' sit right under your thumb.

Fitts's Law is usually taught to designers, but it's also useful for PMs. You can read any screen in 30 seconds and work out what the product team thinks the user should do most often. A 'Buy' button that's smaller than 'Learn more' is either a design mistake or a team that isn't sure it wants to sell. Small targets in high-stakes moments are some of the cheapest wins on any mature product.

A brokerage app has two buttons on the portfolio screen:

— Buy more (120×44px, primary colour, bottom-right, under the thumb)
— Sell (60×16px, text-link grey, top-left corner, next to settings)

Conversion on buy-more actions is 4.2× higher than sell actions on comparable prompts. Some of this is user intent; a large share is Fitts's Law doing its job.

Now compare the same app's 'Delete account' flow: 20×12px text link, three screens deep, with a confirmation dialog and a 24-hour delay. That's also Fitts's Law, used in the opposite direction — and a good UX team knows to use both. A mature product ships a 'destructive action audit' once a year to make sure no accidentally-large destructive targets have crept in.