I think outer keys are larger because the first Sholes typewriters had all round keys plus an enormous spacebar across the bottom. Then subsequent tyewriters ended up with round or rectangular keys, again all the same size, but with a ragged edge at the right and left sides because of the wacky horizontally staggered layout. Then at some point typewriter/computer makers wanted to fill in the extra space and end up with nice rectangular blocks of keys. Some of these designs just used the key top to fill the available space, and some designs made the pressable key-top a square but had a stagger and let a lower part of the keycap fill for the rest of the space (I think partly to avoid the stabilizer problem? that prevents off-center presses). At some point those went out of fashion when people figured out that the associated gaps between the pressable key areas led to people missing the keys, and so keyboards all just ended up with stabilizer bars on the larger keys, instead.
Basically, at least 95% of every popular keyboard layout was just copied straight from the previous generation, and no mainstream manufacturer has really questioned the main design features like 1/2 – 1/4 – 1/2 key stagger, 19mm spacing, large keys on the side that fill all available space in a rectangular block of keys, qwerty or similar key layout, one big spacebar with few other tasks for either thumb.
There have been some mass-market ergonomic keyboards, like Microsoft’s, but they only make a few small changes from the standard keyboard.
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If keys are going to require overall hand movement to reach, then I’m sure there’s some advantage to having larger keys (e.g. tab, return, backspace, control), but I’m fairly convinced that it’s ultimately better to stick to small keys, and just make them all trivially reachable without much overall hand movement.
