You are making nothing wrong here.
When you create your list of 10 millions elements, allocation each node with make_unique everything is fine (Of course the data is not on the stack, except perhaps the first node !).
The problem is when you you get rid of the head of your list: unique_ptr will take care of the deleting the next node that it owns, which also contains a unique_ptr that will take care of deleting the next node... etc...
So that in the end the 10 millions elements get deleted recursively, each recursive call taking some space on the stack.
