Default Why is the apk file so small?

Hi, I'm new to the forums so I'm not sure if I should be posting this question. I just wanted to backup a game because it's 1gb in size and I don't want to download it every time I mess with my phone. I just downloaded an app called "App backup & restore" which backs up whole apps on your phone. It saved it as a 38mb apk file. Can someone explain why is the file, that's supposed to be the whole app, so small and how am I going to restore the app with it? It just seems unnatural to backup a 1gb app to a 38mb file so I thought it's probably not backing it up correctly.