Seeing Huge Amount of Data Usage with Google Music
OK, I think the answer to your question is that Pandora's stream is never more than 64 kbps for mobile devices, whereas Google Music uses 320 kbps MP3s and there's no downsampling.  That's 5 times the amount of data, assuming Pandora doesn't use a better format than MP3.

Since Google Music transcodes your files, I don't think you have much of an option here.