Different Artwork Resolution (Re-dowloading Purchases)

When I purchased music from the iTunes store, the song came with some nice 1400x1400 resolution artwork embedded into it. When I lost the file and redownloaded it, the new song's artwork was 600x600. I am not a fan of this relatively small image size, and no matter how many times I have tried, I cannot redownload the song to have the high res artwork. Can someone explain to me why my redownloaded purchase has different artwork from the original actual purchased song's metadata?