I would say that
Albatrozz had compressed the assets ( images, audios, vidoes ) of this game to the best of his abilities which is why the size is only 518MB ( zip format ). I don't think it can be compressed further without compromising the quality

.
However, for future reference, you can request for the community to provide compressed version
here ( in ccordance to their rules ). Unfortunately, you can't request for this game to be compressed there because it does not fulfill the size requirement ( game needs to be at least 700MB ).
Hope that helps.