I've found out that it only happens, when the zoom level for the page is something else than 100% when starting the preview. Could be interesting if it's the same for Chrome. When the zoom level is at something else, it also happens with Firefox and IE.
Note: the problem is not that it is more zoomed out with smaller zoom. The problem is that coordinates get messed up, proportions get messed up (window size to sprite size).
Example: setting x of the sprites to mouse.x, when the mouse was in the middle of the window and their origin is at the middle of the cube, looks like this at 50% zoom: