I understand the documentation does not make this very clear, but this property is used to return the total used space in the organization. This is why the same number is returned for each user you call it on.
As a workaround, one of my colleagues created the below code snippet to determine user storage usage by adding up the size of all the items they own:
# Accounts is a list of all user objects that you can query
Accounts = gis.users.search(query= None, max_users =500)
# For each user in the org, give me a list of the items they own
for user in Accounts:
storage = 0
userItems = user.items(max_items=100) # The max_items default is 100
if userItems: # Checks to see if the user owns items
for item in userItems:
# Adds up the total storage of all items the user owns, in bytes
storage += item.size
print (user.username + " is using {0} bytes of storage".format(storage))
Feel free to reach out with any questions!
storageUsage and storageQuota