Remember that the UE editor itself is around 15GB which is mostly libraries for various elements inside a game like audio libraries and shader/render libraries. When you build your game, the assets inside your project folder don't includes what's required to run a game and the UE engine has to includes everything required for your game to run without the editor. It's normal that the engine folder is around 220MB by default and could be even bigger if you used some crazy tools in the editor. There's also the possibility that some of the assets which you see as only a few MB were converted into some form of RAW format and this result with files being 3x-4x the original's size.