I haven't read all six pages, so if what I am about to touch on has already been said, my apologies.
I have trained with people of varying heights, weights, and builds, and each presents their own unique challenge. Being tall, lean, and having a fairly high center of gravity (I am long legged and short torsoed), I have advantages and disadvantages against different sizes and builds.
Since we're talking about overweight people here, I'll address that.
In hapkido: When throwing an overweight person, the extra weight does make it more challenging. At my height, I have to generally lower myself quite a bit in order to get my center of gravity low enough to effectively throw them. A shorter than myself and stout person I am more inclined to lock and bear to the ground than to even attempt to throw them.
The advantage that they have is that they're already lower than me and have a built in physical advantage in a throw. The only thing in my favor is that I'm long enough that if they're not careful, they don't throw me, but simply lean me over a bit. We don't do competition in hapkido, but we do some live free play. I tend to keep the shorter, stouter people from getting their hands on me because once they do, I can easily wind up having the disadvantage.
If the overweight person in question is a lady with any strength, her advantage is greater in regards to throwing and being hard to throw, as ladies generally have a lower center of gravity than us gents, even when the height is the same.
In Taekwondo: being overweight means more torso to guard, but since your arms don't gain any length by being overweight, you don't really have any added measure to guard with. Since grappling isn't a part of taekwondo, in competition, an oveweight person is generally at a disadvantage, all things being equal. Since the added weight is fat, it moves them into a weight class that they wouldn't otherwise be in. A five'eight guy against a 6'4 guy in the same weight class and close in weight means that either the tall guy is way underweight (I'm not), the shorter guy is Franco Columbu, or he's obese. Since I have yet to spar or compete against any five-eight guys who were Franco Columbu clones, that leaves one option.
The last time I sparred with an overweight gent who was roughly five eight, I had torso shots galore, wore him down, then had head shots galore. His weight kept him from being able to work any angles to get inside of my guard, so I wound up just picking him apart from a distance, and pretty much at my leisure.
In Kendo: Provided an overweight person's mobility is pretty reasonable, the only thing that they have to be concerned with is endurance. Good management of energy pretty much solves this so long as the person is not in poor health.
No real advantages or disadvantages for the extra weight in kendo. Probably the only real disadvantage may be in buying gear if the person is exceptionally heavy.
Important: I think that it is worth noting that contrary to popular belief, not all overweight people are in poor health, out of breath, or slow. I've trained with some 'fat' guys who were unbelievably quick, very strong, and who had more than the average amount of endurance. There are some heavier ladies that I have trained with who are exceptionally good and used their build to great advantage. This may be the overweight person's biggest advantage: underestimation on the part of opponents. Don't think that an overweight opponent is easy pickings.
Whatever build you are, be it tall and thin, short and stout, tall and stout, or average, the key is to train to maximize your build's strengths and to guard against your build's inherent disadvantages.
Cardio health is the biggest pitfall for an overweight person, but in general, the extra mass does provide extra protection for bones and vital organs, if for no other reason than that it absorbs some of the shock. Somewhere in there is a trade off.
Daniel