At its peak, the Roman Empire held up to 130 million people over a span of million square miles. How could such a powerful empire collapse? Simple, the real effects of debasement took time to materialize... and then exploded.
This is a map of the Barbarian Kingdoms. Justinian claimed the Vandal kingdom his, along with the the Iberian coast (part of the Visigoth kingdom),the Italian peninsula from the Ostrogoths. This resulted in him having most of the Mediterranean coastline.