I tried to backup a big table, 70,000 rows, and the resulting db.zip only contained the first 500 lines. I looked at _backup.class and saw what can be the problem: at line 569 the while() sentence asks for !empty $rows or page == 0 (which is right the first time), but at line 612 you unset $rows and add 1 to $page, so next time $page is not == 0 and $rows is empty!!!

That's why you only get the first 500 rows of the table in the backup file. I think the unset $rows should be first sentence in the while(), before it gets values.