I believe it is just using the browser history to reload the previous page. It doesn't try to retain all the data in memory - just re-issues the query to load the data. You can see this if you have a page with a large datagrid where you get the loading spinner... returning to this page you will see the data loading spinner again.
So, to answer your question, no, it should not have any significant memory implications.
But it can have consequences. Closing a page to often result in the user being logged off from the system for instance. To my knowledge using the 'Show home page' action resets this memory (may be somebody from Mendix can confirm this). So my advise is to try to clean up the screens as much as possible if you can :)
Thanks, that makes sense. The only consideration is then UX, not memory footprint. One thing that I have noticed is that when you close a page to return to the previous, the previous page is not updated with whatever changes you made in the "popup" page. With that in mind direct navigation generally has to be used. Very valuable to know for cancel/abort type buttons however.