This is because release groups are completely, utterly clueless about video. The file size is set ahead of time. Most groups set e.g. "8GB for 1080p movie", "4GB for a 720p movie" etc. in x264. Historically speaking, these pre-selected sizes were designed to fit on different media types, such as CD, single-layer DVD, dual-layer DVD.
Few people use DVDs anymore, but most groups still make files far larger than they need to be.
I rarely download pre-made videos because of this, so haven't downloaded any encoded in h.265, but I suspect they simply chose a smaller pre-set size.
The correct, non-stupid thing to do is to set the quality and let the movie be however large it needs to be, usually under 4GB. This allows more easily encoded video, like CGI films (Toy Story, etc.) to be small while very difficult films (anything with a lot of noise and movement, like war films) are large but don't look terrible.