I think the nerves may just be spread out over a bigger area. Maybe smaller with more concentrated nerves would actually feel better. Like women's clits have all the nerves in a small area, but still have intense pleasure from oral sex.
Another thing to consider is maybe the large penis has larger muscles contracting during the orgasm, and it's those muscle contractions that determine the level of pleasure during the orgasm. So maybe larger penises generally have better orgasms.
Another thing is a smaller penis is easier to fit in one's mouth, so maybe they get better blowjobs in general.
I do know that it feels great to have your cock worshiped by someone that loves to have it in their mouth.

So someone may be more enthusiastic when sucking a big one which leads to more pleasure.