You can diminish the size of the journal by means of these commands:
sudo journalctl --vacuum-size=100M
This will retain the most recent 100M of data.
sudo journalctl --vacuum-time=10d
will delete everything but the last 10 days.
--vacuum-size=, --vacuum-time=, --vacuum-files=
Removes the oldest archived journal files until the disk space they use
falls below the specified size (specified with the usual "K", "M", "G" and
"T" suffixes), or all archived journal files contain no data older than
the specified timespan (specified with the usual "s", "m", "h", "days",
"months", "weeks" and "years" suffixes), or no more than the specified
number of separate journal files remain. Note that running --vacuum-size=
has only an indirect effect on the output shown by --disk-usage, as the
latter includes active journal files, while the vacuuming operation only
operates on archived journal files. Similarly, --vacuum-files= might not
actually reduce the number of journal files to below the specified number,
as it will not remove active journal files.
--vacuum-size=, --vacuum-time= and --vacuum-files= may be combined in a
single invocation to enforce any combination of a size, a time and a
number of files limit on the archived journal files. Specifying any of
these three parameters as zero is equivalent to not enforcing the specific
limit, and is thus redundant.
These three switches may also be combined with --rotate into one command.
If so, all active files are rotated first, and the requested vacuuming
operation is executed right after. The rotation has the effect that all
currently active files are archived (and potentially new, empty journal
files opened as replacement), and hence the vacuuming operation has the
greatest effect as it can take all log data written so far into account.