It seems many people who answered here also want a smaller keyboard to position the mouse closer to the centerline, or make room for the mouse.
Perhaps I should mention my own setup then: I don’t use a mouse 🙂 I use this - rather confusingly named - fabulous trackball: the Kensington Expert Mouse:
This is the best trackball ever. It’s been around for many years. People who like trackballs swear by it, and I can’t do without it.
The trackball is right up against the keyboard. So even though I use a full-size 104-key keyboard, the “mouse” is ergonomically placed and doesn’t require a lot of space.