MeGusta and Im pretty sure all other x265 groups aren't really considered official scene releases and usually the sources are the larger x264 scene releases. I've found that you can get the same if not better results as MeGusta encoding with a simple -cq 27 with the nvenc_h265 encoder which is reasonably fast.

A good portion of the world thats pirating media is playing it cheap junk with 10+ year old CPUs that can't handle x265, most do not have terabytes of media they just watch and delete so overall size isnt a huge issue, most likely when a new codec does become more mainstream, it won't actually mean smaller releases anyway, it will just mean better quality ones.

In the 00's the standard everyone used was 800mb DivX because thats the size CD-Rs came in, over time, going into the 2010s we got x264 releases but the targets were around 4-8gb usually and by that point the size of optical media didn't really matter since flash drives are cheap and reusable and overall internet speeds for people continues to increase as well so its more likely that when the day comes, the scene will probably coalesce around something like 8-16gb per release.