Last year I downloaded maxmind free GeoLiteCity.dat from http://www.maxmind.com website and it's file size around 30MB, but today I was trying to download new bat file from maxmind http://dev.maxmind.com/geoip/legacy/geolite/. But it has only 17MB file size.
So any one know that 17MB file has more ip address than they offered 30MB file?
I can confirm your findings. The GeoLiteCity.dat file has indeed decreased in size (relatively) by a lot. Which seems strange as with the growing IP pool you would expect to see a growth in IP localization data as accuracy improves.
On Maxmind's forum many users seem to report less accurate results in relation to the decrease in file size. One of such topics listed here : http://forum.maxmind.com/viewtopic.php?f=13&t=3820 .
The change in filesize seems to have occurred from February to March 2012. Other then reverting to a version from this period of time, there is not much help I can offer.
Though it's worth mentioning that this file is part of Maxmind's "legacy" geoip databases. They have a new version/format of their software which might offer better results. You can find more about it here: http://dev.maxmind.com/geoip/geoip2/geolite2/ This one is 27MB (unpacked) and likely more accurate.
Hope it helps!