When trying to increase capped collection size from mongosh as documented:

https://www.mongodb.com/docs/manual/core/capped-collections/#change-a-capped-collection-s-size

db.runCommand( { collMod: "log", cappedSize: 100000 } )

An error happens: MongoServerError: Cannot change the size limits of a capped collection.
User is authenticated and can create new collections and modify existing content in other collections.
I have only tested with one capped collection in the newest 6.0.01. The capped size was 1 GB and tried to increase it to 2 GB.