I have a Mac that is running 24/7 for a variety of reasons, but one of them is to act as a media server for an AppleTV. As such, I need to run iTunes at all times so that the Apple TV has something to stream from.
In the past, I could see that iTunes would take perhaps as much as 1.5GB of RAM when left running for days on end. Presumably, it was because of some caching that was occurring with media that was streamed recently. However, after recently upgrading to High Sierra and the most recent versions of iTunes (May 2018), I now see that iTunes is consuming closer to 5GB of RAM!
What the heck is making it use so much memory? Yes, I do have upwards of 30k songs on there (yes, all legit believe it or not), but I have not significantly increased the library size in the last few years and have only really noticed the jump in the last few months. Perhaps file caching associated with a process is now reported for a process? But that would be strange since kernel_task seems to own that.