@nprofile1q... Note that for me Godot reports much lower memory usage than the OS. I think it showed something around 3 GB in Godot, while system reported 5.5 GB for the running app.
Still, there is no good reason for your kind of jump either. What do you have there, just one column with a short string? 1.5 GB on that would be bizarre.
All the data that I add to the Tree is already in memory in scripted objects, and that fits into the first 1 GB. Then the Tree somehow needs 4x that.