Just so you are aware, the Palm V which came out in 1999 had just such a power supply. A bit bigger than 1 inch, but it's a switching power supply and doesn't get warm (i.e. it's efficient).
So did the Newton (1993 per Wikipedia). I remember it as my first-ever encounter with a (small) switching power supply — it seemed unreasonably light as “wall-warts” went.
Just so you are aware, the Palm V which came out in 1999 had just such a power supply. A bit bigger than 1 inch, but it's a switching power supply and doesn't get warm (i.e. it's efficient).