It is being uprezzed. 4:3 is MUCH smaller than 1080.
You need to change your sequence settings to match the 4:3 footage. Probably the easiset way to do this is to make a new sequence, set it up to be 4:3, then copy and paste the entire timeline from your old version to the new.
You may need to adjust some aspect ratios under the Motion > distort tab, depending on your project setup and your FCP version.
In fact there would be an easier way if you were on FCP 6 or better. What version are you on?