itunes files too big with converted songs - any help?

Do anyone know how to avoid having extra files that are converted from increasing the size of my music folder? I mean when one convert to AAC, itunes convert the songs but keep the old and the new files together which increase the size of the folder. Say you have 50g of music and you convert all, you will have 100g. How can I prevent this please? Apple take note this is important.


If any Apple development people reading this, take note of the followings:
1, Volume to be the same regardless - the sound check aloneis not adequate.
2, Files size stay the same after conversion, to allow one to keep original files soemwhere else just in case.