It all depends what you're compressing. Compressing something which is already compressed (Video, Mp3/4, Images) will never work, where-as things like huge text documents etc, will get a good compression ratio.
depends on the compression method and the type of media which you are compressing as SPeedY_B said trying to compress something which is already compressed is fairly useless, also compressing somethings like realworld captured images in BMP format using a Run-Length encoding method can result in file expansion.
Also when you are compressing this 2GB are you wanting all the 2GB back when you decompress? A DVD film can be 2GB in size, but you compress and chuck away enough then you can get it down to 700MB.
I am guessing however that you have 2GB of data or work and so on and want to get it all on a single 700MB CD, are you going to want to access individual files from the collection of 700MB, or are you going to just compress it onto the CD, move it to another system (or the same system after a format) and decompress it all?