I'm not sure where you're getting the idea that data on optical media is compressed.
If you have a 100MB file and burn it onto a CD it's still going to be 100MB on the CD, the only exception to this is if you are using some form of backup software which is compressing the data for you, But standard burning software does not compress data onto optical media.
2TB is a fair amount of data to backup and kinda rules out optical media, you'd need over 400DVDs! So you are going to be limited to hard drive backups (or tape, but the drives are bloody expensive then, i.e. £1000+)
For a semi reasonable price I'd be tempted to invest in 2 of these
http://www.google.co....aw8YnvDA&ved=0CBcQrRI
Stick two x 2TB drives in each one setup as RAID1 (simple mirroring) and copy your data onto each one and store them separately somewhere, you'd then essentially have four copies of your data so even one drive failed in each box you could still read the data.
(you could save the cost of the drive boxes by just copying the data onto four hard drives, but then you'd have to find a decent way to store them, putting them in boxes helps to keep them safe)