I am unable to write to the full size quoted on the disc. Why is this?

There is various reasons why the size of the disc can't be reached.

The main one being confusion over the labelling of discs.

One kilobyte is 1024 bytes, and One megabyte is 1024 kilobytes.

One megabyte on a hard disk is 1024 x 1024 = 1,048,576 bytes.

If you store 700mb on a CD, thit is 700 x 1,048,576 bytes of hard disk space, which would equal 734,003,200 bytes of hard disk space.

We leave a small margin for universal compatibility, so we quote 730mb.

It can depend on the size of files, if you have a few big files, you can fill up the disc quickly, If you have many small files, the index tables will take up a lot of space.

Most writing programs will set aside some space to organise the disc to their liking, this generally causes the disc size to be slightly less than stated.