Memory requirement listed for RHEL 9 (and hence Rocky 9):
1.5 GiB for local media or NFS network installation, 3 GiB for HTTP(S) and FTP network installation
That does not say much about the normal runtime of the system; it only tells something about one application (the Anaconda installer).
The systemd-journald does not write to disk by default. It only keeps log on tmpfs, i.e. RAM.
The man journald.conf mentions more than just SystemMaxFileSize and RuntimeMaxFileSize.
Could it be one of those other “Max” settings (that the 1GB RAM makes small)?
The log message does also point to number of entries rather than size.