Problem

I just installed Windows Photo Gallery 2012 (WPG) since I decided to use it for photo organizing. However, when I apply any type of tags to my photos using WPG there is a significant reduction in the file size of the tagged photos. Generally, there was file size reduction of about 15 percent (while the resolution of the photo was unchanged).

I find it hard to believe that the file size could be reduced by 15 percent without some kind of picture quality compression – and loss of quality – being involved!

Question

Can anyone help explaining the file size reduction I'm seeing? Could it be due to unwanted photo compression? Also, when does Photo Gallery compress photos, and can it happen without my consent?

More information:

For instance, I have a JPG picture taken with a DMC-ZS15. The original file size is 4,67 MB (4.907.520 byte). After adding a geo-tag to the picture the file size is reduced to 3,97 MB (4.169.988 byte). Adding a face-tag to the original similarly reduced the file size to 3,97 MB (4.170.671 byte). Adding other types to tags also reduced the file size.

The "date modified" is updated the instant the tag is added. Since there is no way to undo (CTRL+Z) adding a tag, the damaged has potentially been done.

I found that the file size reductions happen when tagging photos using two different Panasonic cameras (a DMC-TZ8 and a DMC-ZS15) both of which have been my main cameras. However, I also have photos from other cameras (two different Canons and a Windows Phone) that actually saw a slight increase in file size, when I added tags to them - as expected.

I'm using Windows Photo Gallery 2012 (build 16.4.3528.331) on a Lenovo X230, Windows 8.1 with the latest updates. Note that in settings I have set the JPEG quality to 100 %.