Actually, in a real reef, there probably are more predators (pods) to eat the algae than there are in a scrubber, because algae in the wild is not cleaned every week.
As for the giant-scrubber example, it would probably do better than a regular-sized scrubber, because the chlorophyll-to-coral ratio would be similar to a real reef. My calculations showed that a regular 100g tank would need a standing stock of at least 4 pounds (remaining after a cleaning) of algae to equal a real reef. The levels of algae/chlorophyll (both benthic and planktonic) on reefs are well-measured and quantified, and the number of corals/gallon on reefs are far less, thus real reefs have a much larger algae-to-coral ratio than tanks do.