Each document in "Chats" could have a principal that can be shared with a new user at any time but each document in "Messages" would be encrypted using the Chats principal, thereby cutting down on the amount of space required to store the keys. There would no longer need to be Principal keys for each Message document. This would also cutting down on the amount of time it takes to share an entire group of Messages with a new user.
I am not sure if this is possible with the current implementation but my understanding of the code suggests that it is not.
For example:
Each document in "Chats" could have a principal that can be shared with a new user at any time but each document in "Messages" would be encrypted using the Chats principal, thereby cutting down on the amount of space required to store the keys. There would no longer need to be Principal keys for each Message document. This would also cutting down on the amount of time it takes to share an entire group of Messages with a new user.
I am not sure if this is possible with the current implementation but my understanding of the code suggests that it is not.