On the compression ratio of xvidcore

Hello, dear:
   I'm studying to use xvidcore to encode yuv images into mpeg4, But
I find the compression ratio I get is quite low. For example, I use
the following command to encode a yuv file :
p9322@wllab1:~/program863/xvidcore-1.1.3/examples$ ./xvid_encraw -i
coastguard_cif.yuv -type 0 -w 352 -h 288 -frames 1000 -o
coastguard.mpeg4 -max_key_interval 10
then :
p9322@wllab1:~/program863/xvidcore-1.1.3/examples$ ls -lh coastguard*
-rw-r--r-- 1 p9322 p9322  44M 2008-10-31 17:35 coastguard_cif.yuv
-rw-r--r-- 1 p9322 p9322 6.1M 2008-11-21 22:35 coastguard.mpeg4

you see? the compression ratio is only about 7.
this is part of the encoding output info:
xvid_encraw - raw mpeg4 bitstream encoder written by Christoph Lampert
2002-2003

   0: key=2, time=     21, len=   36006 | type=I, quant=  2, len=   36006
   1: key=0, time=     24, len=   19250 | type=P, quant=  2, len=   19250
   2: key=0, time=     28, len=   20667 | type=P, quant=  2, len=   20667
   3: key=0, time=     23, len=   20431 | type=P, quant=  2, len=   20431
   4: key=0, time=     29, len=   22397 | type=P, quant=  2, len=   22397
   5: key=0, time=     23, len=   20005 | type=P, quant=  2, len=   20005
   6: key=0, time=     28, len=   21632 | type=P, quant=  2, len=   21632
   7: key=0, time=     27, len=   21329 | type=P, quant=  2, len=   21329
   8: key=0, time=     23, len=   20117 | type=P, quant=  2, len=   20117
   9: key=0, time=     24, len=   20940 | type=P, quant=  2, len=   20940
  10: key=2, time=     11, len=   36691 | type=I, quant=  2, len=   36691
  11: key=0, time=     34, len=   18852 | type=P, quant=  2, len=   18852
  12: key=0, time=     35, len=   21458 | type=P, quant=  2, len=   21458
  13: key=0, time=     34, len=   20337 | type=P, quant=  2, len=   20337
the resulted P frames are not much larger than I frames.

Then I use xvidcore to encode the webcam stream, the output info is as
follows:
frame: key=2,len=   14441 | type=^@, quant=  0, stat_len=       0
frame: key=0,len=       0 | type=^@, quant=  0, stat_len=       0
frame: key=0,len=    8593 | type=I, quant=  2, stat_len=   14441
frame: key=0,len=    2676 | type=B, quant=  4, stat_len=    2676
frame: key=2,len=   14325 | type=P, quant=  2, stat_len=    8593
frame: key=0,len=    8105 | type=I, quant=  2, stat_len=   14325
frame: key=0,len=    2813 | type=B, quant=  4, stat_len=    2813
frame: key=2,len=   14290 | type=P, quant=  2, stat_len=    8105
frame: key=0,len=    6932 | type=I, quant=  2, stat_len=   14290
......

The second column is the length of coded frame. My webcam is 320x240,
so the compression ratio is about 320x240x3/10000=23. And strangely,
the volume of P frames is even larger than I frames sometimes.

It's said that mpeg4 can achieve at most 400 compression ratio.
Generally, it can achieve 6,70 compression ratio easily. But why the
CR I get is so small ? Any suggestions?

The version is xvidcore-1.1.3, and the OS is debian linux-2.6.21.7.
Thank you very much!!