Agreeing with people that are pointing out it's just the neck. Queen's attack is based on what appears to be a Boa Constrictor/Anaconda, but that type of snake is only deadly because of its crushing power where it uses the weight of it's entire body to snap its prey. But it's top part is much weaker than it's mid part as in several upon several movies have shown where people escape constrictors.
Sanji faced the full mid part, and weight of the entire body which lets Queen put more force into it. But if he's just grabbing Chopper with his neck, liken it to throwing a rope and only able to lasso part of it. It's much weaker and has less of a grasp.