downloading songs to mobile phone

[tinkerbells]

Hi
My son is trying to download songs from Limewire. They start out as a size of around3,000 Kb but for some reason when they arrive on his phone (Sony Ericsson 750i), they end up as a size 4-7 Mb. Does anyone out there know why this is happening and how we can stop it - as it means he can only get 3 or 4 songs on his phone. By the way, these are MP3 files to start with. Thanks for any advice!!