I didn't know that I tunes ever sold MP3s! AAC's that iTunes currently use are 256kbit/sec. MP3's can be encoded for speach intelligibility that have a small file size up to barely compressed. Nearly all compressive ile formats can have the level of compression adjusted such as jpgs for photos.
iTunes should be considering bette quality or lossless downloads IMO but Apple seem in no hurry when 90ish% of customers don't care about or need better quality which is already pretty good.