Like 150,000? (Viacom sent takedown notices for 150,000 clips prior to the lawsuit. I'd say, this is a "significant" amount.)
Weird logic. It's rather the other way around. Because the clips are so popular, they ended up on Youtube. And Viacom (as the rightful owner) clearly sees the presence of those clips as not useful for their business goals (whatever these may be). It's not for Youtube or the individual users to decide what happens to Viacom content. It's Viacom.
How else could they determine what is in the lockers? They have to take a look to estimate the real number of violations, and the real damage.
Ciacom have made clear that they are not going to go after individuals. They are after Google. That's where the money is.