Ah, the dimensions are most likely the problem. I use high resolution monitors, and have always disliked the smaller images, particularly when they get upscaled. May have to do some A/B testing on that point.
I use Lensdump and imgchest as alternative hosting sites for images/sets where I want to preserve large images like the ones for !hegre@lemmynsfw.com , but find that sizing down to 4096 limit is still a good tradeoff. TBH, afaik, nobody here on LemmyNSFW that I know has ever complimented positively about the quality of uhd images... (Btw, I used to mod https://old.reddit.com/r/UHDnsfw/ so I do appreciate the very large ones but its also a very niche audience in these days of mobile devices...)