I'm not sure how compressed the png's that GM puts out, but when you export to google maps format the many many files can take up many many bits. Is it possible to implement a post process compression like http://pmt.sourceforge.net/pngcrush/ or something similar? I know there are some others out there, and You might be able to find one with a free non restrictive license (like freebsd licence or something). Just a suggestion at the moment. I think others have reported 50% reduction in file size when they used it on the output of a different program.