For sure within every dog breed their is a reason they were bred they way they were. Some for size, to be more hardy, to pull larger loads, to hunt bigger game, some were bred smaller, sometimes to better fit that climate, to better fit the terrain, so they could be something that they couldn't get in a large dog, and everything in between.
I'm not too familiar with malmutes so I can't answer anything about them, but i'm sure they were developed for a reason and have their strengths and weaknesses compared with other breeds and we could all argue about the merits of such attributes.
But a GSD wasn't meant to be big, just like a 60lb chi wouldn't probably be a very good representation of the chi breed or a 50lb JRT or a 20lb GSD. For any number of reasons those aren't really good representations of the breed and within these generalizations about bigger and smaller dogs we can always find exceptions on an individual basis.
and size isn't the only determining factor, i've seen plenty of 70lb gsd's that couldn't do squat because of their genetics and upbringing, so obviously size isn't the "only" factor.