Final cut pro X export files bigger than originals

Question

I got two WMV files, each file size about 500mbs, I converted them to mp4 using permute. Afther converting, each file size about 490mbs.
Then I open the mp4 files with final cut pro, I just want to mend the two clips together, it's that simple. I finish puting the clips adjacent to each other. Then hit export and chose encoding method .h264.
When final cut finished exporting, the movie is over 4GB in size.
Why is it so big ? the orginal files combined size is just about 1 GB.

Answer 1

File numer 1
format :H.264, 1,280 x 720 AAC, 48000 Hz, Stereo (L R)
FPS: 29.97
data size: 1.23 GB
data rate: 3.64 Mbit/s
current size: 1,280 × 720 pixels (Actual)
File number 2
format: H.264, 1,280 x 720 AAC, 48000 Hz, Stereo (L R)
FPS: 29.97
data size: 962.9 MB
data rate: 3.62 Mbit/s
current size: 1,280 × 720 pixels (Actual)
After putting two clips together in final cut pro and export with h.264 encoding
Format: H.264, 1,280 x 720 AAC, 48000 Hz, 5.1 (C L R Ls Rs LFE)
FPS: 29.97
data size: 4.3 GB
data rate: 7.10 Mbit/s
current size: 1,280 × 720 pixels (Actual)