Your body is a reservoir of power. So many components go into the force that one can produce in any movement, but that is often overlooked to make it as simple as building more muscle. This is a thought that comes from a simple mind, missing the splendor of the forest because of a single leaf. Your joints, connective tissue, muscles, angles, bodyweight, breath, and many factors play into your power in martial arts, but developing that power isn't complicated at all.
In a fight, your muscle can be a good and sometimes underrated factor in martial arts. If you were a strong person with good technique without much muscle, and fought a more muscular person who also had good technique, the dynamics of the fight can change.
Muscle is good for absorbing attacks, which can be a nuisance when trying to find an opening, and the added weight can certainly add a bit of force to your attacks, depending on how your muscle is trained.
But oh, it goes deeper than that. You can train your muscles to be very dense or even rubbery to the touch, giving strong contractile power even reflecting force like a rubber band. You could also train your muscles to be big and soft as Play-doh, meaning a pretty convenient punching bag for a quick and powerful smaller guy. And you must work to truly train the functionality of that muscle, so as not to fall behind people in your own weight class, let alone those below your weight.
Those muscles have got to work as a unit if you want to be able to take all of your weight and use it in a powerful way; at the same time, there's gotta be some isolated strength, because you'll be taking all of your weight and force to drive it into a single limb.
So training for muscular strength in, say, squats would have to be coupled with pistol squats, heavy bag kicks, or hanging single leg raises. This way, you're able to take a maximum amount of force and transfer it to a small surface area.
And a lot of that force is transferred through the core, so you need to train that to be the strongest you can. I feel that your core, legs, and grip are three things that you should train daily as a martial artist, because they are three of the most essential aspects of your power development in martial arts.
Also, you'll need to know how to transfer that power when your foundation is tired. You won't always be peppy and full of energy in a match or spar, so training your foundational muscle consistently will help you be able to produce heavy force, even when tired. That's partly what made Mas Oyama so fearsome during his 300 man kumite; he himself sustained heavy injuries, but still fought ferociously like a wounded lion. Seeing his incredible tenacity left many of his opponents mentally defeated even before he faced them.
Joint mobility and connective tissue strength are incredibly vital to martial arts skill; imagine if you sat on a swing and pulled it all the way back, but then the chains snapped because they were too rusted. This is essentially what you'll have if you try to transfer that force without the proper support.
I cannot stress enough how important isometric exercises are for developing your connective tissue. In fact, a lot of martial arts training already involves isometric exercises (horse stance, sanchin kata, eagle claw training etc.) and the masters who have trained these for years have tremendous strength in their tendons with relatively small frames.
Joint mobility is also a manner of persistent practice. A big secret of flexibility is that, even without stretching, your flexibility improves a lot when you get stronger.
However, this often requires training your stabilizing muscles like the adductors, rotator cuffs, or erector spinae to improve that flexibility. Furthermore, moving meditations like tai chi really help to open up the joints with their rotating, circular motions. This is fantastic for overall flexibility.
Most overrated however, in my opinion, is the important of breath in martial arts. Your breath is a strong reflection of your emotions (breathing heavy could be agitated or upset, paused breathing could be fear, etc.), which can let your opponent know and potentially manipulate your mindstate.
Your breath is also a strong way to help you produce force; you'll often see a karateka producing a loud, diaphragm generated shout as they punch, and it's stronger than doing the same punch without the shout. Just as important, proper breathing helps you to have a proper rhythm when you fight. You can keep a certain measure of you opponent's style by matching their breathing, or know the right timing to attack or counter attack from observing their breaths.
Now go develop your foundational martial arts power, then practice the basic techniques of your art daily. Your status as a martial arts powerhouse won't be too far away. Your one body has many parts, but when you can effectively unify them, you'll have true power.