They aren't very large birds themselves. So it would stand to reason that the eggs aren't going to be very large. But if it is an unusually small egg, I would wait and see what they do with it.
Sometimes the adults know if the egg isn't going to develop right, so they will ignore and sometimes kill the egg. If this happens, I would simply remove the egg from the cage and discard it.
They'll try again when they're ready. Hope this helps, good luck.