Maybe I was "primed" to think the FOB was really light-weight. But I got a chance to play with it today, and it felt just about right. Any heavier, it would have felt like a couple AA batteries. Any lighter, and definitely it would feel flimsy.
Disclaimer: All my vehicles use a key, and a separate FOB on the keychain for lock/unlock. So I'm not used to the more modern cars that use just a push button start, where the FOB may be more substantial in weight/size. But for me, the Model S FOB was perfect - I wouldn't change a thing.
The Tesla rep there did say that the FOB would match the color of the car, adding one more data point on what others have heard.