Don't worry, the full game's content isn't in the demo; they are similar in size due to system requirements, scripts, sounds, and so on, but the demo doesn't contain the full game's files.
Viewing post in Demo Futa Nights Survivor comments
how? idk alot about coding but there no way the demo code is that big, at that point you might want to redo some to reduce the file size cuz that is super bad for games and game deving in gen, i don't know alot about coding but even i can see a issue with that, if that is the case than you might want to redo some or learn how to make u coding better,
It’s not really about the programming in this case; the thing is, in the demo you can explore large parts of the university—albeit with some rooms blocked off and not much to actually do. However, since it shares the same textures and props, the file size remains almost the same; there’s a lot of cut content, but it barely affects the overall size. Don't worry—we know exactly what we're doing; it just depends on the specific situation. Cheers!