PBase images rarely look good at anything other than Original size, whatever that size may be. That's why I for one would prefer Original to be the default, though that would still cause problems with people who either forget to or don't know how to resize, and post 6000 pixel wide images showing every pore and nose hair of a model.
The key problem with viewing any size other than Original is that PBase's resizing algorithm is pretty primordial compared to something like Photoshop's. A 1200 pixel wide image which is viewed at the 800 pixel large size will lose sharpness. But so will a photo which is resized by only a few pixels because it's the entire photo that is being resized and reprocessed, not just those few pixels.
Your image PalmBeach Waves Crashing in your New Zealand gallery, for instance, has an Original size of 1,280px × 960px. However if you toggle to Auto this becomes, and I quote, "1,280px × 960px (scaled to 1,273px × 955px)". Yes, that's only 7 pixels of width and 5 of height but it does mean that the whole image has been reprocessed to fit that smaller size; it's not just an exercise in cropping. It's not a catastrophic change, but it's a noticeable one.
I think you hit on part of the problem. I just stepped through the paging.v8.js code which is where the "auto_size" action is happening - well actually it is not happening. The code is not working correctly. No matter the window size of the browser the auto_size activates without a proper parameter which ends up resizing everything a fraction smaller. It should be resizing to the window.innerwidth or slightly smaller than it (which might be those extra pixels), but the code is looking for window.width which does not exist in the object. So it looks like it is just resizing to the fraction smaller and not the actual window width plus the extra smaller fraction to bring it within the window width.
Also resizing using the proper tools should not change sharpening - usually smaller sizes actually look a bit better when sized smaller than the device's display resolution. In Photoshop when you downsize say a 12MB image to the device's resolution it looks better than when viewed sized more than the device's resolution. And the softening caused by pbase.com's auto_size is happening on my images that are smaller than 800px wide (
https://pbase.com/richo/image/122524089) and one images that are 720HD (
https://pbase.com/richo/image/168791869) and images that have 1920 as the width (
https://pbase.com/richo/image/168563180). On my website, the code there downsizes everything to 800x600 (which I hate), but it does it more or less correctly compared to what is happening on pbase.com with the "auto_size" function.
Bottom-line, once again code was rushed out to get the pbase.com "mobile ready" that doesn't actually work.