my misconception on mounting

content

my question:

how can /staging have a larger size than /, since it’s a subdir of /?
when in reality, these two concepts (linux dir structure and filesystem/mounting/block-device) are not related in the way i thought
- /staging is a subdir of /, is because of how linux dir is structured
- whether the size of the device mounted on /staging is bigger than that of /, depends on the device itself, nothing to do with whether /staging is under /