Louis has an extremely loyal fanbase, who will buy several variants of one album to help his numbers. Niall does the biggest tours after Harry, because he has more GP interest. And in the US, Niall's albums sell by far the biggest after Harry's. Overall, they've all honestly done quite decently. Liam never had enough interest for albums or tours to succeed, but he had a couple hits. All the others have had big albums and/or hits and/or tours.