In the older versions (as well as 8.1) the preview window resizes itself it 'JUMPS'. When you drag a corner (auto mode) the window gets bigger but the preview stays the same size until you hit the next jump. For example when my preview is at 360x270 it won't get bigger until the next jump, which on my machine is 720x540.
8c is truly re-sizable on a thoroughly variable scale. If I drag a corner not only does the window get bigger but the preview scales up to follow!