I've noticed that most of the fellows who do chins and pull-ups with relative ease at the gym are the slimmer dudes. Less weight to pull up, would be my guess. Form is everything in my opinion. If you can't do these exercises using good form, then start with something where you don't compromise form.
it was that way when I was in the Marine Corps as well. Most of the guys with a perfect PFT with 20 pullups were very small guys. I struggled with 5 in boot and 10 later on. Of course it doesnt help that youre so weak and beaten down all the time.
Its funny, because the 20+ pullup guys always struggled humping gear and ammo, so while they looked great on the physical fitness test, when we actually went out and had to do grunt shit, they were falling out, passing out, unable to hump their load...
i had to help carry a small kid up a hill called the Reaper in boot, he fell out, couldnt keep up, so we split his gear off and handed it to another 'fat body' (military term for anyone over their BMI charts specified weight), and grabbed him and carried his ass uo the hill in addition to all our gear.
Kid was a cocky ass former track stat when we got there, straight out of highschool. Ill never forget the SDIs look and browbeating he gave that kid when he saw us fat bodies cresting the ridge carrying mr 140lb perfect PFT track star. Kid got a brutal gut check.