I know it's a bit late but I thought I'd throw in my two cents...
The law of diminishing returns definitely applies to the benefits of muscle building in sports. When you lift to improve your performance, improving things like your strength, power, and stamina, it is only natural that you will put on some muscle. This muscle can be good since it'll add some pounds of quality weight to your frame while helping to diminish your body fat (since it increases your resting metabolic rate). Also, it aids in the mental aspect of things. If you're looking good you'll feel good.
However, too much muscle and you end up hurting your game. It happens a lot when guys get addicted to the vanity side of it all. They put on all this muscle and sure, they look good, but its all for show. It loses its functionality.
Now as far as Tiger is concerned. Yes he has definitely put on a lot of muscle. But is it too much? I would have to say no. He is still playing just as good as he ever was. And to reiterate another point someone else brought up, he is being trained by top professional trainers. They'll ensure he's in top physical condition for golf at all times.
On another note, I think in today's fitness world there is a huge emphasis on muscle building and that flexibility gets put on the back burner. That's because you can't see one's flexibility but you definitely can see someone's bulging biceps or ripped abs.
That's why I am huge fan of programs like Core Performance. They emphasize on sports specific strategies, not simply weight lifting. Because you have to ask yourself, are you a bodybuilder or are you a golfer.
My dad just bought me this one fitness mat too that I tried out today which is pretty cool. It had all the stretches on them already. I think it was called G2 Fitness mats or something... I can't find the link right now but i'll post it next chance i have.