Mobile product page image gallery – what should be the default zoom state?

For example, there is product page on mobile website or app that looks like this:

User taps on image to zoom it in - what should happen? Should it fit into viewport, like this:

Or should it fill viewport, like this:

And if it fills viewport - how left/right swipe should work? Should it open just one image or the whole gallery?

Or maybe something else should happen?

I downloaded a few shopping mobile apps and they're very inconsistent. Please share your thoughts or studies regarding the matter.