mod_geoip2 by default reads the GeoIP database from disk and only caches to RAM when it's explicitly configured to. Nginx only caches the GeoIP database to RAM, increasing the RAM usage many times over for each worker. For situations where GeoIP data is seldom used it would be extremely valuable to sacrifice some speed to save RAM by caching the database to disk instead of keeping it in RAM 24/7.
See: MaxMind's mod_geoip2 documentation under the : http://dev.maxmind.com/geoip/legacy/mod_geoip2/#File_and_Caching_Directives-3