The World uses a lot of objects under the hood, however a majority of these objects are automatically pooled, which helps eliminate constant garbage churn. All Pooling Options can be found under the General Settings Window of the World.
In some instances, pooling can increase the memory usage of your game. In these situations, you can play around with disabling some of the pooling options to see if it reduces the memory.
You can also play around with this setting (so long as Pool Hierarchy Game Objects is enabled), which controls the number of Game Objects the World will keep pooled for use with Hierarchy Organizers.