White margin around photos when I zoom into Photos app

Well, that was a great answer if you had been using Photos on a Mac to view your pictures-- I missed that you were asking about an iPhone. Sorry about that...

The Aspect Ratio Grid choice on the phone is under the three dots in the Library view. It's the same general idea.

This should work better for you, I hope.

There are two ways to view the thumbnails in Photos. They call them the "Square Photo Grid" and the "Aspect Ratio Grid." You can switch between them with the View menu, like this:

In Square Photo Grid mode, you're given the option to switch to Aspect Ratio mode, and vice-versa.

There is also a button at the top next to the slider that adjusts the size of the thumbnails:

I never use the Square Grid.

Hope this helps...

It is a pity that the "AspectRatio Grid" is not available in all views, for example in the Days or Memories. The cropped and tiled "Square" is making it hard to recognize my photos, when I cannot even tell, if a photo is panoramic, or in Portrait or landscape orientation.

I am avoiding the tiled view whenever possible.