Instructors know it more too well that everybody in the martial arts comes in all different shapes and sizes. Age, gender, height, weight, strength, fitness, attitude, mental and physical ability (have I missed any) all have a bearing on the ability to learn, level of performance and results. Instructors, including students must be made aware of these variables which will be either a hindrance or an advantage depending on what skill they are trying to master. Speaking from experience, traditional martial arts training often demanded an amost similar physical outcome from each student. This was achieved by the natural selection of the few who stayed on to get Black Belt. The rest had to move on. I remember one of my Black Belt's attitude, in the mid ‘80’ with his students, ‘you’ll never be any good, take up needlecraft instead’. I don't think that sort of sarcasm exists today.
Martial arts have come a long way since then, encouraging everyone to take up training and Instructors are willing to help students achieve their goals. This can be done in many ways which does not necessarily has to be solely based on physical attributes. But having said that, I can’t remember any student who has not impressed me with something they can do better than the other person in the classroom. It's all about discovery, students will eventually do particularly well in some specific area of their discipline. I don't know whether they are good at it because they like it, or whether they like it and that is why they become good at it. Maybe, John Will would like to do a bit of research on this.
Everybody in the martial arts, with time, will gain confidence, and become faster, stronger and fitter, that's the way the body works. So there is no cause for concern to anyone hoping to achieve their goals.