> "how much space this will take if I copy it to other filesystem" This is ambiguous between "how many bytes are all these files in total" and "how many bytes

> "how much space this will take if I copy it to other filesystem" This is ambiguous between "how many bytes are all these files in total" and "how many bytes does it take to store a single copy of all these files on such-and-such file system (mostly the current one)". The latter can be different because of transparent compression, which is common on e.g. BTRFS.