You can't directly display a Rectangle as it isn't a Component. I assume
you mean rectangular components of some description that have the panel
as their parent.

You are incrementing the iterator twice in the loop. And apparently not
creating a new one. The enhanced for loop makes these sort of mistakes
less likely:

     for (RectangularComponent comp : componentList) {
         comp.width *= 10;
         comp.height *= 10;
     }

If you have to use Iterators directly, do one thing at a time. Don't
embed the next within a larger statement:

     for (
         Iterator<RectangularComponent> iter = componentList.iterator();
         iter.hasNext();
     ) {
         final RectangularComponent comp = iter.next();
         comp.width *= 10;
         comp.height *= 10;
     }

If you've just poked some unencapsulated variables, then the component
has no way to know that it should be redrawn. A more conventional way to
operate would be to get (a copy of) the component's preferred size, and
set it back updated:

    Dimension size = comp.getPreferredSize();
    size.width *= 10;
    size.height *= 10;
    comp.setPreferredSize(size);

The component now know it is invalid (comp.isValid() will return false).
Also it will have informed the panel it is invalid, and the panels
parent, all the way up.

Once you done all that, call revalidate() on the panel. This will,
shortly after, relayout and then repaint the panel appropriately.

Tom Hawtin