I'm fairly large guy. 5'9" - 10", ~240lb or a hair less. I suck at pullups. I can crank out about 5 good ones, anyday. When I trained them, I was good for about 9 perfect ones. Just losing 10 lb and they get so much easier.
Im not advising you to lose weight. But don't forget that size and strength dont increase, linearly, at the same speed. Since volume is related to the 3rd power of radius and cross sectional area only the 2nd power. Unless you are untrained, bodyweight will always increase at a faster rate vs strength. It's why a 10lb dog can jump 5 feet in the air and a 100 lb dog can't jump 50' into the air.
For example. If a 300lb man, as a 1RM, can lift 50% of his bodyweight (anything, let's just say a 1 arm press for 150 lb as example)... The equivalent feat for a 150lb man (all other things being equal, genetic twin just smaller) would be 73% of his 1rm or a 110lb press. Not a 50% or 75lb press. I'm just using Wilks formula to calculate this...
Or a double bodyweight deadlift is way more impressive for a "big guy" vs a "small guy". The same applies to pullups.
All I'm saying is that you must be realistic in your goal. I don't know how many pullups you can do. But if you could get to 5 or 10, that would be fantastic. 25-30+ pullups is for the skinny dudes.
Regards,
Eric