I came across a weird sentence in a paper. I wonder if its phrasing is correct grammatically since it doesn't make much sense to me.
Since 1991, the number of children with a mother in prison more than doubled, up 131% and children with a father in prison increased by 77%.
I have seen no such usage before where an adjective follows "more than". Is there something I don't know about such structure, or that the author has used a grammatically poor statement?