EDIT: oh no, I thought the problem was in fullscreen, so maybe it is not this what I wrote below /EDIT
English by me, so wording may differ:
right mouse button on the executable (or the shortcut), choose "properties" (in win11 may need : further options -> there: properties).
In the upcoming window, choose the "compatibility" tab, to the bottom is a field "change High DPI settings", that pops up a new sub-window. At the bottom of that, check the field and then from the dropdown list pick "application" (or maybe it says "program"? the one that is not "sth-with-system")
OK, OK ... that's it.
happens with old game engines in highDPI/ resolution monitors, and FullHD already
is high resolution.