Consider merging the statusbar into the headerbar to save vertical space?
When I review photos from a DSLR (which are a native 3:2 aspect ratio) on my widescreen laptop (16:9, 1366x768), in photo view mode (not the browser) I often find myself wishing for gthumb to offer me more of the precious vertical space to let the photo appear bigger on my laptop screen without being in fullscreen mode.
Today, it struck me: the statusbar doesn't seem to have enough stuff on it to strictly require it being a separate widget from the headerbar. Indeed, it only seems to host one button on the right (which could probably be put into the headerbar), and some text on the left (such as "photo number/total - resolution - filesize - zoom percentage"), and that text could easily go under the filename in the headerbar's title section, by using gtk_header_bar_set_has_subtitle and gtk_header_bar_set_subtitle, making better use of that space as well.
So yeah, that is my humble UX and UI compactness suggestion. Reclaiming the space currently used by the statusbar widget would allow a taller and wider photo display, which is really useful, on laptop screens in particular.