Tayshaun Prince, Kevin Martin, Marcus Camby (although injury proned), Chris Bosh, Jamal Crawford, Andrei Kirilenko, Samuel Dalembert ....
A few very effective NBA players who remain skinny. Adding muscle is definitely something Durant needs to do. But, as for adding much weight, it won't be that necessary, especially since he's projected to be a small forward. Look at someone like Tracy McGrady, who still weighs about 210-215. Durant can get stronger without building too much mass. If he gets caught on a switch guarding Dwight Howard, then Dwight Howard wins that mismatch. Switches happen all the time in the NBA. Durant's team will just have to make sure it doesn't happen too often. It's not like Durant will be the first ever small forward to be switched on a power forward/center and not be able to stop him. It happens.
Durant may need to add a little more weight,but it's more muscle/strength than it is is actually adding weight/bulk that is important.