How to trim content from a heavy database
For content, I know two options, both based on Views:


Create a view of old content only, and use Views Bulk Operations to purge it from your site. This is literally what you asked for, but also something I would discourage.
Create a view of new content only, and use Views data export to only export nodes recent enough to matter. That way you will still have old content on site, and only lose it if site crashes and you need to restore from incomplete backup.


Also, I suggest you to evaluate Universally Unique IDentifier, it helps with consistency if you are exporting data individually, not as a complete snapshot of entire site.