Celine Dion struggled to sell records in France. Yes, for real. What may seem absolutely impossible from today point of view giving the insane amount of records she sold since 1995, was true until the release of D’Eux.
Barry White is one of those acts with highly recognized hits that never sold large amounts of his studio albums, making him one hell of a candidate for big selling compilations. At that point, the main argument to get into million sales area is the cannibalization factor. In other words, the least compilations have been released, higher sales of those available will be.