CD's have two ways of being burnt. Type one is an MP3/WMA disc which will only play in PC's, DVD players and other MP3/WMA compatible players. The limit to the disc is in mb - in most cases 700mb.

Type 2 is an "audio CD" where you create a disc just like a normal audio CD - play it on any CD player at all. The limits on this type of disc is in minutes - in most cases it's 80mins. I think you are creating audio discs, so you are being governed in minutes, rather than mb's