Which audio format, it can take up less space?

I have an ordinary mp3 file (any singer) about ~2mb. I want to convert another audio file. And it needs to work in this format. So, after convertion this file, it will be least, smallest size in pc. It doest not important to used techniques, programming languages, software etc. Only in terms of ideas, is it possible? I am looking for researching to Comparison of audio coding formats in wikipedia. But, it is so complex.

for example, 
mp3 (~2mb) to amr (~210kb)
mp3 (~2mb) to acc (~480kb) etc..