My Mods v4

My question is, I am living in Hong Kong and there are several sound segments in asf file format. I intend to convert the asf file format to MP3 file format, so I am able to keep the sound segment to my Sony Clie and listen while I go to job. However, when I convert a 50 minutes asf file (almost 4 mb) to MP3 file, it turns to 50 mb after it is transformed to MP3 file format. I want to know the factor the file size turn very big and is it possible to remain the same size after the converting?
Thanks very much to all people and once you are able to offer me a hand on this, like asf converter softwareⓘ, it definitely helps me a lot!!!!