In its basic form, Fitts’s law says that targets a user has to hit should be as big as possible.
Dear god, my biggest beef with using a smart phone is that UI designers 1) love to have tiny buttons for shit, and 2) the tappable areas for those buttons are almost never made larger than their tiny graphics, so it’s a bitch to actually tap them.
I used to be a mobile app developer, and when I wrote apps by myself I would always expand the tappable areas so they were easy to hit with fat fingers. My last job was working for a huge cable company (maybe the name rhymes with “bombast”) and whenever I expanded the tappable area of a tiny button the UI designers would pitch a fit and insist that that not be done. Management would agree with them on the grounds that expanding the tappable area would require too much time to implement - and then they’d order me to spend even more time un-implementing it.