MPPlantOfficial Android system WebView is a kind of browser which is installed separately. It does not include into your APK, that's why the APK size is lower if you build it without Crosswalk. The point is that if you export with Crosswalk, your app will be launched in that Crosswalk browser which is included in your APK. So it will be the same browser for all devices which means your game will behave the same way on each of those devices.
Now when you export without Crosswalk then your game will be launched in the system's WebView. In other words, your game will run in the browser which is installed on the particular device. Now each device might have different version of WebView installed. Different versions support/not support different features so there is no guarantee that your app will behave the same way on every device. I would even say, it is guaranteed that it will not.
Now regarding Android 5+ which everybody talk about. Shortly saying Android 5+ WebViews support most needed features and C2 apps should work well and you shouldn't need the Crosswalk any more.
My personal opinion after some tests is that Crosswalk is still more safe to use and has better performance that native WebView.
Thank you very much!