[CentOS] wtmp binary is 5 times bigger than its equivalent text file

Sun Jun 22 01:24:48 UTC 2008
Centos <centos at unixplanet.biz>

On one of our servers wtmp is growing very fast.
The binary file is about 50M but when I change it to ascii with fwtmp command,
The ascii file is only 10M.

binary file is being 5 times bigger than text file ?
Any suggestion ?