Export size has very little to do with resolution.
It has EVERYTHING to do with the codec you use.
When you import footage into FCPX, you typically import footage from a source which use a codec employing inter-frame compression, I.e. only a number of key frames are actually recorded, and only the delta (or change) from one to the other is used to describe the frames in-between. It saves significantly on storage, but relies on a lot of processing power to replay.
Footage of this nature is difficult to deal with on an NLE such as FCPX, as it requires constant recoding of the individual frames not recorded by the source. Hence, FCPX "optimizes" the footage and recodes it into an INTRA-frame type codec, I.e. it keeps the keyframes, and creates actual individual frames for each frame not recorded in the original, but described through the codec. The file size is much larger for this type of codec, but requires little processing power.
This speeds up the editing process as the CPU does it have to deal with complex decoding all the time, and can focus on the tasks during the editing process itself.
Now, when you export a movie in FCPX, make sure you don't export your footage using any of the Apple ProRES codecs. These are interframe codecs, and are used for editing purposes only.
Export your footage to compressor, and select something such as h.264 with settings that matches your source.