Games keep getting bigger and bigger. This game is expected to be about 100GB, and that's not uncommon for modern AAA games. The CoD games have been over 200GB for a while now. Previous FF games have been similar size. RDR2 was 120GB.

I would expect most people playing FF16 on PC to have a small SSD drive with their OS, key programs, and maybe a couple of games, then a HDD for bulk storage.

I'm not interested in the FF series, but if I was this message from the devs means "clear up some space on your SSD". Which can sometimes be an inconvenience.