I think this is because the node itself is also size 1 and does not have any children. It can however have children. I'm actually not familiar with Quick-Union Weighted but if it's bit like the other union find algoritmes I've seen you can for example do
quw.union(0, 1);
ID: 0 0 2 3 2 2 6 2 8 9
SZ: 1 1 4 1 1 1 1 1 1 1
quw.union(0, 2);
ID: 2 2 2 3 2 2 6 2 8 9
SZ: 2 1 6 1 1 1 1 1 1 1
So now 0 en 1 have merged and the entire tree starting from 0 is merged with 2 again, still making the subtree starting at 0 size 2.
Like I said, I'm not sure it that's possible in Quick-Union Weighted but the reason for the '1' is still because it's also size 1 on its own.