Clarification needed on Memory Used vs Disk Usage
Jamroom Hosting
As you say, the Disk Usage is what is posted on the site - all your user posts, uploaded photos and media etc. - whilst the Memory Usage is 'RAM' and is what is used by the operation system for temporary storage and to actually run the server. That usage figure will be more dynamic and will change dependent upon site usage etc.
If your sandbox installation is no longer being used (it was set up in the early days when we started developing your site) then yes, delete it and free up the 1GB of disk space that it is using.
hth