Welcome to Android Central! Is this the stock Gallery app, or the Google Photos app? Are you referring to photos that are locally stored, or in the cloud? Are they super small only in the thumbnail view, or also when you try to expand them to full screen?
If you are in the Gallery app, you can use the two finger punch to zoom out. So to make the pictures larger, place two fingers on the screen close together and then spread them apart. You can go from having 8x13 to 4x7 to 3x5 on the number of thumbnails this way.