[gh02t]

Some of the new ultra-compact chargers are using switching regulators based on GaN transistors, which are a fairly new technology. These are more efficient and hence can be smaller for the same power envelope. Some of the Anker products use these, not sure about the one you linked.

But similar logic applies even for the ones that aren't using GaN. More efficient parts and/or designs allowing for more and more shrinkage. One of the other big drivers has been manufacturers starting to market monolithic chips that manage all the aspects of power regulation and USB-PD in a single/small number of parts.