OpenRefine does not use much RAM at all when it first starts and you see the Project screen in your browser showing the listing of previous projects and tags.
It does much more RAM once you click to open a previous project which will open it in memory and includes all previous transformations done on that project.
Many folks will load one of their large projects and just do an export on their project that has been cleaned and transformed heavily into a format (CSV, TSV, etc.) of their choosing and then Create a new Project in OpenRefine with that newly exported file. Then they will delete the older project if not really needed for provenance.
Doing those steps would basically throw away the History of Changes you made in the original project.
Again, memory is really only used once you open a project and see the data grid. If you open several projects in different browser tabs, those all will use memory at the same time, and will only release the memory back to your system (the Java heap) after about 1 hour (maybe it's 30 minutes now? forgot) of inactivity on that project.