All other things being equal, it is physical size.
You can make an analogy with many other things. For example, car engines. Bigger engines are more powerful than small engines when both engines are of the same type. However, if one engine is normally aspirated and the other engine is turbo-charged, then the smaller engine may be more powerful.
Thus it is with power supplies. If two PSUs are of the same basic design, then to get more power you have to make it bigger. But for example, if one PSU is based on a normal mains transformer and the other one is a switched mode supply, then the switched mode supply could easily be much smaller and lighter.
Designs can also become more advanced over time. So for instance, modern PSUs are often smaller than older ones, even if they are both switched mode designs. This can be achieved by using a higher switching frequency or other more advanced features.