Starting a separate post to make this a clean discussion / possible solution. Apparently lot of developers still have the issue of Wix site not showing in full screen mode (with whitespace gaps on either sides of the site) using android webview.
We had exact problems with our sites not showing up full screen in webview. After doing lot of research (including on this forum & other places) and experimentation, the following is what worked for us. No need to do anything on wix side.
Include the following in .java file;
WebSettings settings = webView.getSettings();
Good luck and hope this helps