It’s like a traditional database in this sense: everything that’s in memory is on disk; not everything on disk is guaranteed to be in memory. If you have less than 41Mb of data, everything will be stored both in memory and on disk. If you have more than 41Mb, all will be on disk, some will be in memory.
The maxSize parameter is used to configure how much memory is used, not disk. On disk you’re only limited by how much space you have.