If the "window" size is 720p it doesn't matter how big the layout is. It's still only 1280x720 pixels that will be printed on screen.
@jobel Since Construct's performance was awful when I first started developing, I felt myself forced to use 720, so yes. Also due to how memory bloated big games become, I was forced to stay at 720 in order to not use waaay to much memory. Too late to fix it now.