That is normal. You set the maximum memory Photoshop can take and Photoshop will gradually use up to that maximum.
Chris Cox , an ex Photoshop guru and engineer, once explained that Photoshop retains the memory to speed up processing. It will give memory back if demanded by the OS.
You don't need top do anything.
Yes, this is by design. As long as you set the memory allocation sensibly, leaving 2-6 GB for other apps and processes, it's not a problem. It's just perceived to be a problem when people expect memory use to go down.
Do note, however, that plugins like ACR run in their own address space, separate from the main Photoshop app. So factor that in.
Bear in mind it is virtual memory. This is not at all the same thing as RAM. Almost all the memory given to Photoshop may be swapped out to disk when it is idle, leaving all RAM available. This will only happen if other apps actually need the RAM, not earlier to look tidy.
Thank you very much for for your answer. Since now Photoshop is not the only program behaving that way (the same Google Chrome does - just puts everything possible to cache) I am just curios how the programs decide who gives the system some memory back when it needs it for some other processes. And from the general slowness of everything (after together they eat up to 94% of my RAM) I have a feeling that it takes time for them to decide on that :).
Free RAM is wasted RAM - right? It doesn't do anything good just sitting there. Ideally it should all be in use at all times.