https://en.wikipedia.org/wiki/Portable_Network_Graphics#Comp...

What doesn't change is raw data. In general maps are not like photography, maps are simplified diagrams so that things can be easily distinguished, and that makes them much easier to compress using lossless methods, so using a reduced amount of colors will not be really noticeable, especially if colors are properly chosen.

Look at the palette of an OSM tile: https://i.imgur.com/ecaP17l.png

Using a 8 color palette instead of 256 on xnview results in a 50% reduction. A 16 color palette results in about 40%, but the result is much better. So it does save memory but not that much.