Making a previously non-fulscreen application fullscreen

I was looking around online and I wanted to clarify something.
I have an application that generally launches in windowed mode, but I want to provide an option to launch it in fullscreen mode. Hwoever, when it goes fullscreen, items in screen draw in the smaller format, it doesn't scale with the window.

From what I've seen online, you need to code that behaviour in by hand, but I wanted to see if there was perhaps a way or a silly mistake I made that would make it perform that behaviour automatically? automatically adjust for screen size? is that even a thing?
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