Script to group artboard contents, centre and resize proportionality

This should do what you want (iterate artboards, group, centre and resize):

var doc = app.activeDocument;
var ABs = doc.artboards;
for (var i = 0; i < ABs.length; i++) {
    app.selection = null;
    ABs.setActiveArtboardIndex(i);
    doc.selectObjectsOnActiveArtboard();
    // group
    var o = doc.groupItems.add();
    for (var j = 0; j < app.selection.length; j++) {
        app.selection[j].moveToEnd(o);
    }
    var b1 = o.visibleBounds;
    var w1 = b1[2] - b1[0];
    var h1 = b1[1] - b1[3];
    var b2 = o.geometricBounds;
    var w2 = b2[2] - b2[0];
    var h2 = b2[1] - b2[3];
    // centre
    var ABR = ABs[i].artboardRect;
    o.position = [(ABR[0] + ((ABR[2] - ABR[0]) / 2)) - (w2 / 2), 
                  (ABR[1] + ((ABR[3] - ABR[1]) / 2)) + (h2 / 2)];
    // resize
    w = doc.width - (w1 - w2);
    h = doc.height - (h1 - h2);
    var f = (w1 > h1) ? w / w2 * 100 : h / h2 * 100;
    o.resize(f, f);
}