Thumbnails - some are smaller than they should be

[Fledi]

Some thumbnails don't use the whole space even if the file has the same dimensions, dpi, format as an other file.
I have saved the file again and reloaded but the thumbnail is smaller than the others.

I have set the thumbnails to 300x300px and most of the thumbnails are correct but some have only half size.

[Bhikkhu Pesala]

Please attach the original images.