@hopeco There are a lot of reasons it would be harder for someone, but saying that a heavier person has less lean mass is a pretty large generalization. OP on this post weighs 15 lbs less than me, and is taller. I can knock out 17 chest to bars despite being at a higher weight, because I likely have a lot more muscle.
I find that people get really negative when it comes to pullups. They say they can't do them because they're too big. That's not true. They can't do them because they don't have enough muscle per lb, particularly in their upper body, and they don't have the movement pattern trained. There are plenty of small, thin women at my gym who have trained for months and can't do a single pullup. At the same time, there are plenty taller women, with a BMI that classifies them as overweight, who can do several.
The truth is, most women without life-long upper body training will have to work hard as hell to get a pull-up, regardless of size. Size isn't the major issue. Muscle mass per lb, and the ability to utilize it is.
Without knowing the background of this woman, you cannot say she'd have to lose weight and train harder than a smaller woman. If she's someone who lifts at a high level, she could very well have the muscle mass. If she isn't, and is carrying excess body fat, and little lean mass, than like any untrained person, it's going to take a lot of work.
It also doesn't make my statement less true. Every lb of her has to pull 1 lb. This is true of every person, at every weight. The higher weight isn't the issue. The amount of muscle per lb is.