NW.js is a so called "wrapper".
It "wraps" a GUI around your project to make it run natively under an operating system.
When exporting to NW.js, the actual project (e.g. Sprites, events, etc.) don't make the most part of the size.
The biggest part is the Chromium part.
NW.js uses Chromium, which is basically the Google Chrome browser in small.
So when you run an NW.js export, it basically starts a minimal version of the Chrome Browser and opens your project in the browser.
Chromium is what makes up most of the size.