On the Dutch Wikipedia since yesterday some users complain about that images that use thumb (without ..px) are shown smaller than the thumb size. The problem seems to occur on files with a width smaller than the height. There the thumb (size) is used for the height in stead of the width which causes files to be much smaller than before. In the past the thumb (size) is used only for the width and never limited the height.
Not all users seem to have this problem.