

Blu-rays are compressed. “Zipping a zip file” doesn’t apply here because zips are lossless. Video encoding is almost entirely lossy, and there’s a lot of tradeoffs to be made between file size and quality. The whole point of the more efficient codecs is to minimize the quality tradeoff. There’s also a bunch of parameters to tune the resulting bitrate which is the #1 factor in deciding the final filesize.
That being said, I’ll agree that the least quality loss will come from using a Blu-ray remux since those are very high quality.
MicroG requires signature spoofing, which Graphene deliberately does not support. It is more secure to run the real Play services in a sandbox that forces it to be a userland app than to run MicroG as a privileged system app with spoofing.