Meridian recommend adding additional storage when disc gets more than 80% full. The system needs space for log files, housekeeping, transcoding and for storing playlists and saved focus'. I don't know if with larger storage the recommended free space remains proportional to disc size or is more static. I guess it depends how much transcoding space is needed.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).