`CREATE EXTENSION postgis` implies that you have PostGIS installed. PostGIS is about five times the size of Postgres itself, and has about twenty requirements, some of which have their own heavy trees of recursive requirements, some of which are near-impossible to build in certain environments (e.g. macOS Homebrew) without weird finessing. The (alpine!) Docker image of a PostGIS-enabled Postgres is 713MB. That's a lot of extra binary if you just need to record some points!

And the real problem with all that isn't the size/deps themselves, but rather the fact that the size/deps will probably lead to you wanting to use a binary distribution (or pre-made Docker image) of PostGIS rather than treating PostGIS as any other Postgres extension installed against a regular running Postgres cluster. Which then means that you'll need to find/settle for the set of extensions that are packaged for that binary distribution, rather than just using the standard set of extensions that come with a standard Postgres distribution (e.g. the PGDG Postgres apt repo, which contains many individually-packaged extensions.)