This is currently a known limitation of this question type. We (the Open University) is currently planning some updates to the various drag-drop question types to get rid of these fixed upper size limits on the background image and draggable items. However, given the way Moodle development processes work, that won't land until Moodle version 3.9.
Oh nice. I just started building quizzes and saw a lot of potential in the question type, but was immediately surprised at the background resolution. Can I take it that this fix would close https://tracker.moodle.org/browse/MDL-57842?
That's the plan.