OpenLDAP comes with a lot of optional modules to implement enterprise directory features. You don't need to use them all. You can build a minimal OpenLDAP system that's dedicated to user authentication that runs in under 4MB of RAM, with full TLS/SASL/GSSAPI security. You can't even do that with SQLite, the SQLite binary alone is 2MB and that doesn't give you any network interface or security features.