I wasn't sure of how to ask this, so the title is a bit confusing, sorry.
My question iis as follows: I mounted, on /mnt two filesystems. One was a big NTFS and the other a not-so-big ext4. Inmediatly I received a warning telling me that the free space on / was running out. I realized that this was because it was counting the files on the mounted ext4 filesystem as part of /. Strangely, however, this didn't happen with the NTFS filesystem, nor it happened when I unmounted both of them and mounted them again on / (instead of in /mnt).
So, just out of curiosity: why did it just happen when the ext4 mounted on /mnt and not with the NTFS or the two of them on /?
PD: Suggestions on how to better phrase the question in the title would be thanked.