Different size mysql db backups

Hello,
From my hosting, I made backups in three different ways, and for all of them, the file sizes vary between each other, even for the same database. I even opened the sql files and even the number of lines are different. Why ?

I obtained database backups such as:
-using ftp, from mysql backups folder
-using php my admin
-using backups link and then clicking each db one by one

I called my host and they said the db constantly updates itself based on the processes happening. Now my websites are really simple wordpress sites and very very low traffic websites. But i do know that i have bad malicious content in my databases placed by bots before wwhich needs to be cleaned. Is that why these backups i obtained only 5 minutes apart differ from each other? is that why all that virtual traffic is happening ? is that why on my site stats I see I/O usage 0% one minute and then close to 100% the next minute? number of processes and physical memory usage also varies from minute to minute greatly. I have really simple sites with at most wordfence plugins and akismet as the most fancy plugin and that is all.