There is a downstream bug opened for a potential enhancement that the gpk-update-viewer should not always launch a maximized window, especially on a large monitor (4K 65 inches monitor):
It looks the "-1" as a width intentionally passed to gpk_window_set_size_request makes the window running with "small form factor mode" in most of the cases:
I am actually not 100% sure about what is the best way to handle here, especially for hi-dpi large screen. Perhaps there is a chance to make a choice of initial window size by checking practical screen size. Also, "Remember" the last closed size of gpk-update-viewer might be a candidate choice as well.
We moved the upstream bugtracker to GitHub a long time ago. If this issue still affects you please re-create the issue here: https://github.com/hughsie/PackageKit/issues
Sorry for the impersonal message, and fingers crossed your issue no longer happens. Thanks.