Only tested the editor behavior regarding texture resources, I can confirm that once texture been loaded into editor for editing, it will stay in memory until editor restarted.
- Fresh opened editor, texture memory usage is normal
2) Open or simply right click on 100 textures, texture memory usage up 400M, and stay that way
3) Hacking engine source to force GC those in-memory textures, texture memory usage back to normal
It seems that behavior is actually by design (to boost editor performance by minimize loading time is my guess), so it might be too much work to change it in engine level. I would image a resource tracking plugin would be the solution: to track and unload resource when necessary.