When running

rsync -Paz /home/sbird "/run/media/sbird/My Passport/sbird"

As told by someone, I run into a ran out of storage error midway. Why is this? My disk usage is about 385 GiB for my home folder, and there is at least 800 GiB of space in the external SSD (which already has stuff like photos and documents). Does rsync make doubly copies of it or something? That would be kind of silly. Or is it some other issue?

Note that the SSD is from a reputable brand (Western Digital) so it is unlikely that it is reporting a fake amount of storage.