Hey, @gbultman 
Great question. You are not missing an available endpoint. The endpoints you found are designed to give you the current state of the list, such as its current size and the contacts that are currently members of the list.
To track the list size, you’ll need to look at alternatives, such as setting up a cron job to call the API daily and store the data in an external DB, for example.
Best,
Jaycee