Global Thumbnail Settings don't effect existing thumbnail proportions

When you change the default properties for thumbnail size, only the newly attached thumbnails will be effected. The size of the thumbnails that are already part of your space will remain unchanged.

In order for you to enable the already existing thumbnails size to be changed, you need to first remove them from thumbnail directory under Confluence Home "thumbnails" directory. Then reattach them again after you changed the thumbnail with and height under Administration > General Configuration > Attachment Settings.