Well I'd wager they drafted Augustine in part because they aren't sure about Cossa and wanted to have another top level prospect in the pipeline. Cossa has all the size you could want, but he struggled in the A last year.
6'1" is anything but undersized for a goalie, I really didn't like how one of the primary things the Holland regime focused on was size. They would have never drafted Quinn Hughes despite the fact he played in their backyard and clearly had absurd amounts of skill.
Size isn't the be all and end all and Augustine's size will never be an issue.