Widget: Drawn Boxes on the wrong location


I have a problem, drawing boxes and lines to the correct position.

My project needs a minimap, where I want to draw the necessary parts by myself. These shall be enemys for example.
I had to override the onPaint-function to draw lines and boxes. Example-part of the blueprint can be found in the screenshot bp_viewportsize.jpg. As you see, the box should be at the very bottom right corner (1280720).
But it is not, see screenshot viewport. Instead it is at 853
480. That is exactly 2/3rd of the wanted size.

I don’t understand, where that difference comes from. Can someone please help?