PSQL incomplete backup : how to debug
Did you get any error messages on dump, or on restore?  If not, then the most likely explanation is that nothing is wrong.  There is no tight coupling between the size of the database and the size of the dump.  For example, if you deleted a bunch of data and did not do a "Vacuum full", then the database itself is unlikely to shrink, but the dump (and the restore of it) will be smaller.