Neither was Chris Pratt, who I would have never pictured to look muscular for GotG, yet the picture of him showed that he looked pretty damn awesome. Most people thought Ledger didn't make any sense at all for Joker, and he turned out great.
I don't really think of Christian Bale as physically intimidating either. But he played it great (and frankly, not many people were too fond of him being chosen in the first place)
I think it is rushing to judgment, and I think it will very likely turn out just fine