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.