The following link shows my current(!) problem best. It’s from another user, but he described it perfectly:
The CANVAS thinks my screen is 1280 px although in reality it really is 1600. So everything is displayed 25% larger. When I zoom out my browser to 80% the website is displayed as it should. However of course I cannot ask every visitor to zoom! Please advise since this is a bug that I can’t solve on my own.