It appears you are only viewing a thumbnail of the picture, hence when you enlarge it it looses definition.

Right click and select properties and see what their file size is.

I can only say what I do. My flatbed scanner, when you put a photo on it for scanning, usually automatically puts a box around the photo and scans just that. For the normal range of photo sizes I use 300 dpi as that is good for printing. However some scanners do not auto detect where the photo is so you end up scsnning the whole area (platen) of the glass on the scanner - in my case 297 x 216 mm. If you dont notice this and have set the scanner to scan at , say, 75 dpi, the resulting "photo" will be A4 size with an enormous white border. Have you a scanner - if so there should be an "advanced" option where such settings can be seen.

I found a somewhat similar problem where a rather non tech friend had scanned a photo but forgot to ensure the scanner only scanned the actual photo rather than the whole platten, so it was surrounded by white - the actual photo therefore, was not a good one as the pixels were scattered over a A4 size or more and the photo part had too few.

I have opened one of my small photos in Paint ( A free part on Windows 7 and, I think, XP). Looked fine. What Operating system do you use?