Why do I get a backup file that has three different sizes? I am using Drupal 7 site on a remote server (bluehost). I cleared the cache before the backup. 


When I am using the backup and migrate module, and use admin -> config -> back up - the size of the file is 456 kb.
When I am using the my php admin interface, and Export the specific database - the size of the file is over 22 MB.
When I am looking at the database interface of bluehost, it's written that the same database is over 176 MB.


I know that I am missing something, but I can't figure out the answer.

What is the difference between the databases? Which one should I save?
456kb does sound very small, maybe you don't have much content/field data? Backup and migrate will exclude data from core cache tables, and the sessions and watchdog tables (though not cache tables installed by contrib modules) by default; so that might account for it. It's also probably gzipped.
An export through phpmyadmin will include all those tables (and perhaps isn't gzipped), so it should be larger than the backup and migrate version
The size of the database on disk, on the server, isn't and can't be proportional to the size of an SQL dump file. There are indexes and other meta data which take up space, so it will almost always have a larger footprint.


You don't need the cache table data, so as soon as you've confirmed that the dump from backup and migrate has the content you need in it (always a good idea to check), you should use that one going forward.