Pullups are hard because they force you to lift your body-weight from day one. Barbell exercises like are easier to scale. If you can’t Squat a weight that equals your body-weight, you simply remove plates from the bar. Worst case you start with the empty bar. But the only way to reduce the resistance on body-weight exercises like Pullups is to lose weight. This isn’t practical. So you need other ways to make Pullups easier.
There are a couple of approaches you can use to increase your pull up numbers. The first thing I would suggest is to focus on getting your reps up on chin ups. Since they are easier, most people can do more. Work your chin ups until you can comfortably do, say, 50% more than you can do now. Then, go back to pull ups. You should not only find them noticeably easier, but the disparity between the two should be less. Generally, you don’t want to do these every day. Depending on your ability to recover, I suggest trying every other day incorporating them into a workout. I do 4 sets of 6-12 reps, and I find dips to be a great companion exercise for pull ups. If keeping dips in this rep range is too easy for you, you can try adding weight and/or working on your shoulder mobility to the point where you can go to the bottom of a dip (where the front of your shoulders are practically level with your hands). Take caution because again if your shoulders aren’t conditioned well for this then you can hurt yourself. Always listen to your body. But, again, adding weight and simply bringing your upper arms to parallel is still effective.
The heavier you are, the more weight you have to pull up and the harder Pullups will be. This is why gymnasts are almost always small. Smaller usually means a lower body-weight. This gives you an advantage on body-weight exercises like Pullups where your weight acts as the resistance. But it doesn’t mean you’ll never be able to do one Pullup if you’re big. It will be harder, sure. But you’ll get stronger if you do the work.
Now, you may be wondering, where do you go from there? Simple. Like any other exercise, when it gets easy, you need to add weight and keep progressing otherwise you’ll stop or getting stronger. So, in order to progress with a body weight exercise like pull ups or chin ups, you increase the weight by adding more weight to your body.