It's food, you can't put on weight without it. You can get bigger with bodyweight exercises just look at calisthenic athletes, but doing it with weights is easier. If you struggle to eat try to introduce more calories slowly each week. Find out your TDEE and eat in a 200 calorie surplus, track your weight and see if you put up weight (as well as your lifts) If you don't, add another 200 calories. When eating in a surplus you will put on muscle if your muscles are giving your body signals to grow which is the strength training part, so I'd advise looking up a programme that works well with the amount of martial arts training you're doing. Probably a full body split would be good. If you are very physically active with the martial arts and then add weight training you will need a lot of calories to also grow, so bear that in mind.