Hard drives are way bigger, but M-Disk is more reliable for very long term storage.
It is a convention that optical discs are discs, and floppy disks or hard disks are disks. It is not a hard-and-fast spelling rule, but since there is indeed a set of official standards for Compact Disc and Digital Versatile* Disc, you can be sure that any conforming optical disc won't be spelled with a "k".
* Were you surprised that the "V" doesn't stand for "Video"?
In another post of yours in this thread you mention that you have gone "all in on M-Disc" for your archiving, which come with much lower transfer rates (they typically max out at ~16 MB/sec last time I checked) and the requirement of actively managing disc swapping along the way. I'm sure you can appreciate the far more efficient option of a slow 2.5" SMR drive.
Enterprise grade, CMR 7200+RPM hard droves, on the other hand, are closer to ten times faster than uray, so more worth it.