Quite probably this is due to the format you are working with in FCE. DV or another compressed format I presume. If the final output is to the telly (DVD or tape) it probably won't register as a problem as the limitations in the format won't really be to obvious here. (The DV format is created with standard TV-definition in mind).
If you're planning to output to QT you would probably have to change the sequence-settings in FCE. I'm not sure what choices you have in Express, but if Apple's 422-format is available it is very good.