It has the "Look Inside" tag. And it's not free. I never noticed that before. On that page, it looks like it's free. But that's only for the Kindle Unlimited subscribers. Under it, in small print on this page, it says you can also buy it for $3.99.
It shows how often I look for books on Amazon!
Amazon says for best quality, the longest side should be 2500 pixels. I just changed my image to that and it changed the width to 1563 (which is the 8:5 ratio Amazon suggests).
That's because you're maintaining the same ration (6"x9"). When I resize my books for Amazon, I switch to Amazon's non-novel cover size (designed to fit their readers, rather than the actual books). Photoshop or Affinity resize the entire image, so you don't lose any detail.
So I'm fretting over nothing. I'm following Amazon's suggestion. So maybe the bigger image doesn't matter when you do the Look Inside.
I think you're confusing double-clicking the image (expand view) with the look-ahead function (which I did when responding to this thread).
People always need to keep in mind that Resolution & DPI aren't the same thing.
Resolution is for screens, DPI is for printing.
This isn't as true as it used to be. Nowadays, with HD 5K screens, 300dpi covers show off an amazing amount of detail. The devices generally control how much detail to reveal, resizing for each device on the fly. For first glance, you get a very low-res image. But you're only allowed to submit one cover image, so you submit the HD version. The 300dpi works best when opening the book on your device of choice (rather than at the sales counter).