Hello and welcome.
Your blog is pretty interesting and makes some good points. Certainly there are karate-ka who are pretty bulky and they still seem to be able to perform karate very well. Some examples include:
Michael Jai White:
Jean-Claude Van Damme:
Note I said perform karate, as in demonstrate certain karate moves in a choreographed way. How these guys would fair in sparring matches I'm unsure about.
In terms of sports performance, an athlete should do strength and conditioning that assists their performance. Their primary focus won't really be how they look. Someone like Fedor Emelianenko may not look like Lou Ferrigno, but he is a highly fit and effective combat athlete.
If I were interested in improving my karate, I would look in to various strength and condition exercises to help performance and prevent injury.
In regards to power generation for blocking and striking, excellent body mechanics wins the day. There is no point in weighing 200 lbs if you have poor body mechanics and can't generate a lot of force.
As to why grapplers have thicker/heavier bodies, well, in my experience heavier people are harder to move. Simple as that. Generally speaking heavier people usually have an advantage in grappling.