Recently, I trained my department in defensive tactics (DT). DT is essentially martial arts for police officers. A lot of it involves modified moves from various disciplines, but mostly from Aikido, Judo, Muay Thai and Krav Maga. I spent the whole day training, but I only had coffee for breakfast and an apple for lunch.
And yet, by the next day I felt and looked more cut and more muscular, despite training on empty. I noticed this in previous classes that I've instructed, and I always found that I swelled up in muscularity after a full day of wrestling people on the mats. And even though I was sore, it felt good. I felt like I had gotten a good workout.
A Finnish study found that wrestlers, bodybuilders, powerlifters and weightlifters all hypertrophy. They just do it through different types of training. Here's the thing: a lot of martial artists strength train to help them in the dojo or ring, but bodybuilders can use martial artists to help them get bigger. If you wrestle or grapple, as long as you eat well and have a healthy dose of T in you, then you will gain some size and muscularity. Grapplers and wrestlers subject their muscles to prolonged tension when they train and spar. You're essentially working out, but instead of dumbbells and barbells, you're using an opponent.
It's different with the striking arts like Muay Thai, Karate and Kung Fu. Explosive strikes create extremely high intramuscular tension, but that tension is so brief, that you wouldn't get big off of it. Whereas the grappling arts are analogous to powerlifting and bodybuilding (moderate but prolonged muscular tension), the striking arts are analogous to Olympic weightlifting (extremely high but brief nerve force).
But I've met quite a few strikers who took up weight training and gained muscle at a faster rate than the average gym rat. It's as if the explosive striking primed their nervous systems to recruit more muscle fiber, so that when they took up weight training, their bodies were primed to lift more weight and gain more size. If you take up martial arts and you want to maintain or even gain size and muscularity, then I suggest you cut back on the frequency of weight training (2 days a week) and abbreviate your program to heavy basic power and Olympic lifts. If your martial arts class puts you through intense warm-ups and workouts, then forget about doing additional cardio: you're already doing it in the dojo or ring.