If the external page fit to the browser viewport, it should have a responsive design. I gone through that url, in that the middle div is set with fixed width and height of 340x380 px. you can only enable the zooming in in app browser.
Also phonegap allow you to execute some scripts after loading the url, just like injecting scripts.
var iabRef = window.open('http://sg-prod-mobilityapi.cloudapp.net', '_blank', 'location=yes');
iabRef.addEventListener('loadstop', replaceCustomStyle);
function replaceCustomStyle() {
iabRef.executeScript({
code: "var itm = document.querySelector('#Main div');
itm.setAttribute('style','width:100%');"
}, function() {
alert("Element Successfully Hijacked");
});
}
Try this and it is documented here