Usually when an application is being updated, its the case that files (resources in apk - code, libraries, assets, images etc) are being replaced rather than being added. Of course the question, is how big is the portion being replaced, which utimately determines the final size of the "update".
If new resources are added by the application developer without removing anything in the previous version then it will likely be bigger, and if some resources are removed and replaced with less, then its the other way round
If the update has contains more resources than before, it uses more
space. If it's a bug fix (similar to a typo in text), it might even
use a few bytes less space.
For most applications, updates will usually increase the total size to a larger value.
But in your case with Google Maps, it seems the items that replaced ones in previous version are "compressed" i.e much smaller.
Uninstalling old version and clean installation of newer version
When a user application is installed, it usually keeps its data (settings, databases etc) and also cache in the /data partition and cache partition.
Uninstalling the application also deletes this related data.
If you then clean install it, you'd also note size difference (even say the were supposed to be same size apk!) Just as Izzy wrote in comments. In this case the clean install will seem to provide the user with a smaller sized update than updating straight (but its somewhat misleading)
But as expected, the size will also increase as new data and cache are saved.
Note: with this route you likely lose all saved data within the application.
Generally speaking though, uninstalling the old version and installing the new version will use roughly the same space as updating the old version.