A muscle exists in the body. All muscles have a very poor mechanical advantage and have to produce very large forces to move joints and do work at distance. When you have a smaller muscle working with very poor leverage - say a deltoid in the shoulder - you do not need much resistance at the hand to exercise the muscle.
At first I saw resistance bands as inferior to heavier weight exercises, my ignorance. Now I see them as very useful and especially useful for the many smaller muscles that I've learned are important for performance and to reduce the risk of injury.
If you want to add muscle mass to your thighs then I would squat with free weights or do leg presses. If you want to condition your rotator cuffs for strength and endurance especially to prevent shoulder injuries then very light dumbbells or cables, or resistance bands are better than heavier weights. Also, there are exercise that if done with a lot of resistance or weight will bypass the smaller muscles (like the rotator cuffs) and instead use bigger muscles. That's counter productive.
(I believe that I tore my rotator cuff (supraspinatus) by doing heavy cable external rotations at the gym. 25lbs? Later I learned that external shoulder rotations for rotator cuffs should not be done with heavier weights.)
I don't know the type of muscle fiber in the various muscles. Some muscles have more of a support, hold position, isometric type function, the abs for example. Maybe these don't have as many fast twitch muscles. ?
I believe one of the best exercises to introduce yourself to the value of conditioning a smaller muscle with resistance bands is the Clamshell. The first reps are not bad but at 2 or 2.5 minutes you will know what your gluteus medius is. It is also fast to respond with results including some hypertrophy in 4-5 weeks.
Gluteus Medius function & the Trendelenburg test (pelvis stability on one leg)
You can buy a 5' Theraband length or a 12" loop. I use a 12" blue and red loop together for Clamshells. I also used to do 'firehydrants' with the same 12" loops, an exercise with detailed motions as specified by my physical therapist. Also 'Monsterwalks' using an 8" black loop (tied). I don't think that they sell black
For rotator cuff external shoulder rotations I use a long yellow Theraband. I warm up with it less stretched where the force is low and then move away to increase the resistance. Because it is long the force can start out high and increase as I externally rotate my shoulder. I put a towel between my upper arm and body as usually recommended.