PS memory issue.

[Lancero]

It used to be that when I would close a PS document a corresponding amount of memory would be freed up.  Now, when I close, the memory used remains the same, with each additional document eating up more space.  I can only process three or four photos before having to close PS and reopen to get some memory space.  My iMac has 16gb of memory.  I don’t know what changed but it’s getting really old.  

Any suggestions about settings that I could change? 

[JJMack]

That is surly not the way Photoshop manages memory on Windows.  Photoshop normally returns memory it has allocated and is managing  when Photoshop terminates.  However there are UI option you  can use to ask Photoshop to return some resources it has allocates and is not currently using. Via Photoshop menu  Edit>Purge>resource. 

[TheDigitalDog]

Not sure what you mean: "when I close, the memory used remains the same, with each additional document eating up more space".

You are not 'closing' Photoshop, you're closing the document right? The actual RAM (you're checking in Activity Monitor) or the scratch disk?

See: https://helpx.adobe.com/photoshop/using/scratch-disks-preferences.html

And:

https://helpx.adobe.com/photoshop/kb/optimize-photoshop-cc-performance.html

By default, Photoshop uses 70% of available RAM

[D Fosse]

Short answer: this is by design because it's faster. Photoshop has always worked like this.

This is why you set a limit in preferences, so that you don't choke the rest of the system.