How would I scale every object to some percent?

And to refine this approach that Larry mentioned:

do the grouping of the objects layer by layer, so you will not have all the objects in one layer after ungrouping.

To start  all that, you do not work directly with the page object, but with the window object and set its activePage to the page you want. As a second ingredience of this recipe we have the "activeLayer" object of the window object.

var myDoc = app.documents[0];
var myActivePage = myDoc.windows[0].activePage = myPageThatIsChangedInSize;
//Loop through all the layers of "myDoc":
for(var n=0;n<myDoc.layers.length;n++){
    var myActiveLayer = myDoc.windows[0].activeLayer = myDoc.layers;
    //Case 1: there are more than 1 page items in the active layer:
    if(myActiveLayer.pageItems.length>1){
        //Unlock all pageItems:
        myActiveLayer.pageItems.everyItem().locked = false;
        //Add a group with all page items on the active layer:
        var myGroup = myActivePage.groups.add(myActiveLayer.pageItems.everyItem());
        //myGroup.scale();
        //myGroup.ungroup();
        };
    //Case 2: there is only 1 page item in the active layer and you cannot group 1 single item:
    if(myActiveLayer.pageItems.length==1){
        //No need to group something, just scale:
        myActiveLayer.pageItems[0].locked = false;
        //myActiveLayer.pageItems[0].scale();
        }; 
    };

As center of the scaling you have to use the center of your active page.
Caution: With that approach all objects on the pasteboard are also scaled.

Of course you can further refine the script and store the IDs of all locked pageItems in an array for locking again after scaling them.

Hope that helps…

Uwe