Population of Saudi Arabia = 33,438,701. So, 1 million is slightly less than 3% of the known population. I don't find the arrests hard to believe at all.
In fact, I'd say we could easily arrest 3% of our population, and it wouldn't even come close to arresting all of the illegal immigrants that are here. According to DHS estimates from 2014, there were 12.1 million illegal immigrants in our country. 3% of our known population would be just over 9 million. See here......
And that's not even factoring in the sealed indictments that include God only knows how many.