The disc capacity decreases when I format a CD-RW/DVD-RW/DVD+RW disc.

When a CD-RW/DVD-RW/DVD+RW disc is formatted using InstantBurn 3, space is needed for the link blocks utilized in packet writing as substitute blocks in case of errors, so the disc capacity decreases after it is formatted.
Capacity after formatting (for your reference):
A 650 MB CD-RW disc capacity becomes approximately 530 MB
A 700 MB CD-RW disc capacity becomes approximately 570 MB
A DVD-RW/DVD+RW disc capacity decreases by approximately 5 MB