Is miniaturization of office device A/C adapters feasible?
So, first of all, cost is a very important factor limiting the shrinking of power supplies. 

For example, an Apple iPhone and macbook chargers have relatively high power density (power going through the power supply per volume, in this case), but that came with a high engineering price, and expensive components and manufacturing processes. 

Of course, there's theoretical limits. There's only so much power you can push through the magnetic core of a flyback transformer per time. More importantly, isolation distances still have to be kept to avoid danger.

Mainly, this is a question of using better controllers, higher-quality switching transistors and transformers to minimize size, along with investing more to make components withstand the larger temperatures inherent to reduced cooling surface. 

As at least better controllers and switches are semiconductors, they still tend to get cheaper over time, and hence, more devices can economically feasibly have more efficient power supplies. Efficiency implies less needed cooling surface, so that might allow for smaller supplies over time.