Backup File Size Discrepancy

Good morning I have a question regarding backup file size discrepancy please. I have a number of clients whose website I manually backup using the BACKUP functionality (not the backup wizard). However, I have noticed that the filesize of the backup can vary greatly from month to month. For example I backed up a client's website and noticed the size of her public_html is 1.138 GB When I downloaded the backup files the numbers didn't add up. I assumed that the files were zipped and hence smaller in size. Account Backup - 661,761KB Partial Backup - 652,039KB MYSQL Databases - 2,290KB & 662KB Email Forwarders - 1KB But then later in the day I decided to run another backup and the numbers were as follows Account Backup - 340,369KB Partial Backup - 302,623KB MYSQL Databases - 2,310KB & 662KB Email Forwarders - 1KB 24 hours later I ran the backup again and these are the numbers I got Account Backup - 1,099,866 Partial Backup - 1,099,466 MYSQL Databases - 2,463KB & 662KB Email Forwarders - 1KB So now I'm worried that the backups that I have been doing have not been complete. In the past whenever there was a network failure, the file would say Unconfirmed xxxxxx.crdownload. And so I would just redo it, but the files look like they have been fully downloaded. So I am wondering what would be the reason for this discrepancy? My internet speed is pretty slow at the moment and I am wondering if I get disconnected from the CPANEL after a certain period or if there are any settings that I need to ask my hosting company to adjust.