Just a question, when I want to open an image in fullscreen or zoom into an image, the image loads twice. Why is that?

Here is an example:
https://streamable.com/ide8d

You'll see it loading once (blue loading circle, grey background), but I can't zoom in or view the image in fullscreen. So I have to tap the image which leads to next screen (white loading circle, black background) where it's loading again. It's much more noticeable on higher quality photos.

I'm just looking for an explanation as to why it has to load twice. I even tracked my data usage in this video (the photo is about 21MB). At first I thought the first time it loads might be a thumbnail version of it, so it would consume less data and the second time would be the full version. But as you can see from the video, it's actually loading the full image twice.