Is there an optimal physique for karate?
I've always tried to develop a 'karate body', that is a physique which is optimal, for my execution of karate techniques (both performance and application). So much so that I foolishly crushed a disk in my spine, at 12 years old, trying to strengthen my body with heavy weights. This injury was the worst in my karate career, and still limits my motion. Thankfully it has not limited my karate, except in regards to some warm up/stretching exercises (i.e. - forward bending can put me out for weeks, so I simply avoid it, and spend more time on other stretches). Strangely enough, this has never hindered my karate, except when instructors have 'forced me' to do bending stretches.
Functionally looking at karate, and in particular, its focus on ikken-hissatsu (the single 'finishing' blow), we must maximise our impact power, but balance this accordingly with speed. Hitting hard but slow is limited combatively, particularly when one is specialising in 'percussive blows'. And being super fast, with insuffient impact power, is completely useless for traditional karateka (typical of many karate exponents, with too much focus, on the sporting aspect of the art).
This balance must be decided by each karateka, via serious training, and self-evaluation. Sorry if this is not PC, but generally speaking, ''fat people'' don't make good karateka, and at best, seriously limit their technical ability. To me, keeping in shape is fundamental for budoka (unless you are doing Sumo), as it results from training (that is 'really training'). People with two or more years of regular dojo practice, who are fat, simply aren't training with enough intensity. Regardless of dan rank, we must then question such peoples karate. In the picture above (tensing up to show some muscle definition), I am slightly overweight, sitting at around 72kg (last Winter).
More important than these exercises is specificity, that is, to supplement your dojo training with regular home practice of kihon. In particular focus on what your instructor is correcting in the dojo. For example if trying to improve your mae geri, try three sets of 20-40 repetitions with each leg. Make sure you can see yourself, to ensure you are not grooving a bad habit into your technique. Such training is karate specific and therefore develops a physique
Obviously diet also plays an important role in training, however I won't cover this here, as I am not qualified to comment (and have a very unorthodox system). In saying that, I'm the first to admit that I eat 'what I want to eat', and reduce my food intake when I detect weight gain. This usually comes as a result of not training hard enough (often influenced by the weather or my hectic teaching schedule - both of which are BS excuses). Personally, as a 100% karate-man, I would prefer to eat what I like, and train more with more vigor, thus killing two birds with one stone.