If I mount sda2 to a directory on the main micro sd card file system [...] will each file take up the full space of the current file system?
No; *nix derived operating systems (such as GNU/Linux, the umbrella under which RpiOS/Raspian falls) organize mounted filesystems in a hierarchical tree structure with a singular root -- hence what you've referred to as "the main micro sd card file system" is usually just called "the root filesystem". The term root here is unrelated to the superuser
This is possibly confusing to users coming from Microsoft Windows, where filesystems are conflated with the devices on which they are stored.
This is why full Windows file paths start with a drive letter (
C: etc) whereas on linux a full path always begins with the root,
Point being, presuming the fundamental root filesystem was mounted from
/dev/whatever, when you do this:
mount /dev/foo2 /home/bar
The OS does not copy the filesystem from
/dev/whatever. In fact, if there are already files in
/home/bar, those remain untouched storage wise, but they become inaccessible until the
/dev/foo2 mount is removed. This means you can't accidentally loose things by making a mistake with a mount point, but OTOH it is a possible source of confusion if for example on a Pi the boot partition is unmounted, and an update puts stuff in
/boot -- because the first partition of the SD isn't mounted, it gets put in the
/boot directory on the second partition and will never be used when booting.
Make sure you understand that last part WRT your backups. If you are using a mount point directory that doesn't contain anything when nothing is mounted there, be careful you don't back stuff up to it when nothing is there because not only will that end up on the SD card: It's also separate from the real contents of the backup device and so prone to end up hidden when you do properly mount the device.
That screw-up is actually hard to make once you understand the point. As a further note/hint: Don't assume everything in the root filesystem is an actual lump of data stored on the SD card, or any storage device. It is possible to mount other filesystems over a network, and commonly file paths are used as intefaces to the kernel (eg., nothing in
/proc is actually stored on a disk, these are virtual files, hence usually reported with a size of zero even though when you read them there may be lots there).