mounted ISO; What determines available room?

[Harry Putnam]

Once an ISO file is mounted for use like in an install ISO, what
factors determine how much room is available?

Example of what I mean with the ubuntu focal fossa  20.4 iso:

inside mounted ISO:

df -h

Filesystem      Size  Used Avail Use% Mounted on
udev            2.4G     0  2.4G   0% /dev
tmpfs           494M  1.1M  493M   1% /run
/dev/sr0        1.6G  1.6G     0 100% /cdrom
/dev/loop0      1.5G  1.5G     0 100% /rofs
/cow            2.5G  104M  2.4G   5% /
tmpfs           2.5G   16M  2.4G   1% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           2.5G     0  2.5G   0% /sys/fs/cgroup
tmpfs           2.5G   84K  2.5G   1% /tmp
tmpfs           494M  8.0K  494M   1% /run/user/999
tmpfs           494M     0  494M   0% /run/user/0

Notice root shows 5% used and a Size of 2.5G

How would someone making this iso arrange things so that the size was
7GB for `/'  Or any other size for that matter.