According to documentation Kentico creates export tasks when an object is deleted. I'm 100% sure how it works, but I'd like to ask if you're using export at all? If you do not envision a need for an export you could disable logging into that table in Settings.
Another option is to process necessary task, so system deletes records, or simply clear it if you don't need to process them.
Other thinks those might cause DB size growth:
- online marketing activities tracking - see best practice here
- storing files into a database
Hope that helps!