We all have this same problem
Apk: crosswalk 17Mo + c2 game 2Mo = 19Mo in the market
When the app is started, 19Mo * 2 or more in the device memory
Like you said, sometimes more than a 3D game
It's a problem of Webview android component.
All the HTML apps like Construct2 export are displaying in a the java Android webview.
A webview is like a browser, but a very light browser. So all the complicated code (html5, Jquery..etc..) are bad or not at all understanding by the webview.
So we need to use Crosswalk, who is a webview with a full browser.
If your game is very simple, you can to try to import it into apk android without crosswalk, I've tried it.
But some android devices can't open it or the performance will be bad.
For example, the function "Load image form url" don't work well without crosswalk.
The game is started on some devices (not all), but the images are not loaded
With Android 5, I've read that the component Webview is better (like a full browser) but I haven't tested it.
If it's true, when all the device will work on android 5 or more, no need to crosswalk