Because the string is JSON, the easiest way to handle the data is to parse it into an array of objects, update the values, then output it as a string again.
// Parse
var container = JSON.parse(yourString);
// Get and update
var i, len, top, left, width, height;
len = container.objects.length;
for (i = 0; i < len; i++) {
top = container.objects[i].top;
left = container.objects[i].left;
height= container.objects[i].height;
width = container.objects[i].width;
// * Save top, left, width somewhere. *
// Multiply by some factor.
container.objects[i].top *= factor;
container.objects[i].left *= factor;
container.objects[i].height *= factor;
container.objects[i].width *= factor;
}
// Convert to string again.
theString = JSON.stringify(container);