By default, notebooks are given 500M (including swap). This can be changed by editing /app/data/customconfig.py.
c.Spawner.mem_limit = '1G'

To apply the configuration, restart the app using the Restart button.

Info:

Remove existing notebook containers
For the memory limit to take effect, you have to remove any existing docker notebook containers using the /app/code/remove_notebook_containers.py script. Notebook data will be intact despite deleting the container.