The "database size" is going to be your data directory, MINUS your pg_wal directory, MINUS unlogged/temp relations, and MINUS things that are not needed for recovery (i.e. all the small pg_ directories such as pg_subtrans). The "database backup size" is how much of that 3GB is part of *this* backup - for a full backup, the number will be the same, for diff or incr, it will be a lot less. The "repo backup size" on the last line is the compressed size of the previous "database backup size"
How to correlate the pgbackrest ( info) database size, database backup size and repo1 backup size and actual du output on DB cluster.
You cannot, really, without deep knowledge of things like which files on disk map back to unlogged tables. However, du on the datadir minus all pg_ dirs should get you in the ballpark. As a quick example:
cd $DATADIR
du --summarize * | awk '!/pg_/{x=x+$1}END{print x}' | numfmt --to=iec
