iphone memory

I've just used up most of my memory on my Iphone and started reconciling my memory. The thing I found is that if i delete a playlist that on itunes takes up 2 gb, then it frees up around 3.5gb in memory on the iphone. Does anyone know why this happens? More specifically does creating a playlist take up twice the memory on the iphone and if so, is there anyway I can more efficiently use the memory - ie not create playlists?