I need a program which could decrease the size of my mp3 files
 

Easy CD-DA Extractor 7.5 is excellent for almost all types of converting / ripping.

you can try
a) reduce the bit rate (upto 160 should sound good. 128 is pushing it)

b) make it joint stereo instead of full stereo (reduces stereo field) i personally hate this.

c) change it to different format? AAC / OGG / WMA ? I don't like any of those though.