There is no substitute for quality being large file sizes only compromises.
First the question is what do you want the Exported file for? Generally when making the QT files out of FCP you want to just use “Current Settings” which means your timeline setting, This is just a copy of the final edit. So if you are editing DV it will be DV. If you are going to DVDSP the file can be encoded before hand or in the program, not sure about iDVD but you can bring in the Current Frame movie into iDVD, the only codec that ends up on a DVD is m2v (mpeg2)
Stuff like H.264, Mpeg4, Mpeg1 are final delivery codecs that are highly compressed and are used for posting on the web and CDs. (For the most part). You probably need to do some reading of the manuals for explanations. May want to look through Compressors help just so you get a handle on this.