Hello there,
It is as intended by the developer, as the main focus is to have them look to have a polaroid style as a series of pictures one after the other as 3 or more pictures occupy the screen at the same time. The images are not small, as already shown in other posts, its not huge but it has a good size per space covered in total on the screen. Pixel art style does not look great in big resolutions for this kind of pictures. Last, at this stage in development it would require immense amounts of work, and this is a project done by a single person. If you want to say that you don't like the artistic way of displaying the pictures because you would rather have full screen pictures that's ok, that's what an opinion is, but don't advise there is "no need" or that it affects performance.
You can insist on it if you so desire as that is your way of thinking, but don't expect a different response.
Have a nice day.