Backup 7.3 was 19GB, restored on 8.0.3 was only 4GB

I can't see anything wrong with my database, but I was wondering what it
might all mean.

Under my 7.3 database, before upgrading to 8.0, I did the following:

vacuumlo my_db
reindexdb -a
vacuum analyze;
pg_dump --blobs -Fc my_73_db | gzip > dbbackup.gz

The resulting dbbackup.gz file was 19GB.

Then I moved the dbbackup.gz file to another system that had 8.0.3
installed:

gunzip -c dbbackup.gz | pg_restore -v -O -d my_80_db

When I ran that same set of commands against the 8.0 database, the
backup from that system was only 4GB. I did note a large number of
large objects were removed in the vacuumlo command, yet I can't really
see any missing blobs, though I'm not sure how to absolutely check this.

Is there anything between the two version that would explain why one is
so much smaller than the other? Is there any reason why the vacuumlo
under 7.3 didn't remove large objects that were then removed under 8.0?

Thanks,
David