I mean yeah if the area had any changes made since the last update (which is always the case) then you have to redownload that areas files.

For the app itself to figure out what changed, it would need to look through the entire new 92GB of data which would require downloading it anyways.

You could make the server compile a list of only the changes that were made since the last update, but that doesnt work because people will often skip a map update which would cause the changes made in between to be missing. So the server would have to store for every map version ever the difference between it and the newest version. Version control software for this type of stuff exists (like git) but its made for plaintext files in the KB-MB range not GB.

Another way would be to cut the map into even smaller chunks in the background (like square kilometers) and change the app and server so it only downloads chunks that have seen changes since the currently stored version, that would reduce it down quite a bit, but it would still be many GB of data because the majority of data in OSM is contained in high population density areas which also see the most frequent changes. This is still a lot of extra effort for the volunteers running comaps, but would be cool.

Over all though i think what you should do is just not download the entire world map every update. And instead just the areas that you actually need.