I also just experienced the memory leak in 4.9.1. UE4 was running in the background for the last few hours without me using it and then I wondered why my tabs in chrome are loading so slow, so I opened the task manager and saw that UE4 used 10.5GB of RAM, so my 16GB were full. The regular RAM usage for my project is about 3GB to 5GB.
Is this leak already fixed (in master branch or 4.9.2) or is Epic still investigating whats causing it?
Maybe this thread should be moved to the feedback section?