Less things to connect, less switches, less caps, ... I think it's mostly about manufacturing cost. If you build a keyboard yourself, you're glad if you have to solder less. So for sure any prototype of a new design will start small. If you go full size price goes up a lot.

For usability alone those small sizes don't make much sense in an office or on a desk.

> For usability alone those small sizes don't make much sense

They do. Having every key reachable without hand movement is quite comfortable. Switching to and using arrow keys is as fast as typing a capital letter with shift.

But it's not everyone's cup of tea, just like Dvorak seems to be great for some people here and pointless for others.