That's because your Windows app is most likely VCL based and does not have a huge overhead on trying to built all of the GUI parts itself.
On macOS the GUI parts are in Firemonkey (FMX) and that is an abstraction layer that does all of the drawing of the controls by itself instead of using the native controls as supplied by the OS.
This creates some extra overhead. There's pro's and cons here. Personally I'd rather had they used native controls instead of FMX as that would look a lot better.
But it is what it is.
From there the app doesn't grow as fast, so it is not all bad.
edit: Just looked at the GUI on my app [0] and without any resources it comes in at 16.2MB (so my estimate was a bit on the high end of things)
On macOS the GUI parts are in Firemonkey (FMX) and that is an abstraction layer that does all of the drawing of the controls by itself instead of using the native controls as supplied by the OS.
This creates some extra overhead. There's pro's and cons here. Personally I'd rather had they used native controls instead of FMX as that would look a lot better. But it is what it is.
From there the app doesn't grow as fast, so it is not all bad.
edit: Just looked at the GUI on my app [0] and without any resources it comes in at 16.2MB (so my estimate was a bit on the high end of things)
[0] http://www.vimalin.com