there’s more that can be done with css. I was able to make the images taller,but they were expanding to fill a space that was taller than what was visible, essentially cropping the image.
There may be other things that can be done to mitigate that and other issues I ran into but I didn’t want to start going down that rabbit hole.
And some of it might be tied to Fancybox’s javascript code too. And know very close to nothing about that.
Unless you’re clever with javascript and css (or someone clever happens along), I think you’re going to have to live with what Fancybox does.
Fancybox does have a Full Screen button that appears on desktop browsers but not on mobile devices. There was a discussion about that here. That might point you in a direction.