Since Plasma 5.10.95 (Plasma 5.11 Beta) the pictures in Gwenview on Wayland are only shown in the left bottom corner probably due to the automatically applied scaling factor of 2x on my 4k screen. The pictures only have the size of one quarter of the screen. Even in fullscreen mode this doesn't change. And in fullscreen mode the task panel is not hidden.
I think this is fixed in Plasma 5.11.
If you can confirm that, would you mind closing the ticket?
Plasma 5.11 fixed this issue.
Plasma is unrelated to Gwenview. You probably also updated to Qt 5.9.2.